After updating Exchange 2019 to CU9, ECP returns "500 Unexpected Error"

You can try to recreate the ECP virtual directory:

https://theitbros.com/recreate-owa-ecp-virtual-directories-exchange-server-2016/

Please follow ONLY the steps mentioning ECP

Are you on coexistence with other exchange version?
Did you try with the followiing URL?
https://<Exchange Server 2019>/ecp/?ExchClientVer=15.2

Check if you are also getting event ID 4 in the application event log like below:

Log Name:      Application
Source:        MSExchange Control Panel
Date:          
Event ID:      4
Task Category: General
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      xxxxServer.xxxx.local

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If yes provide some detailed information of the event.

Sorry I don't see how to reply to individual comments on this system.

I tried MAS's suggestion of going to https://<Exchange Server 2019>/ecp/?ExchClientVer=15.2, but get the same error.

Saif Shaikh, I do have errors with Event Id 4 in the log.  Here are two that are generated when I try accessing ecp:
Current user: 'domain\username'
Request for URL 'https://exchange.fr8.vu:444/ecp/default.aspx(https://exchange/ecp/)' failed with the following error:
System.TypeInitializationException: The type initializer for 'Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor' threw an exception. ---> System.InvalidOperationException: Cannot load Counter Name data because an invalid index '' was read from the registry.
   at System.Diagnostics.PerformanceCounterLib.GetStringTable(Boolean isHelp)
   at System.Diagnostics.PerformanceCounterLib.get_NameTable()
   at System.Diagnostics.PerformanceCounterLib.get_CategoryTable()
   at System.Diagnostics.PerformanceCounterLib.CategoryExists(String machine, String category)
   at System.Diagnostics.PerformanceCounterCategory.Exists(String categoryName, String machineName)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor.LogActivity(CounterType counterType, String organizationName)
   at Microsoft.Exchange.Management.ControlPanel.RbacContext.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.get_Session()
   at Microsoft.Exchange.Management.ControlPanel.AuthenticationSettings..ctor(HttpContext context)
   at Microsoft.Exchange.Management.ControlPanel.RbacModule.Application_PostAuthenticateRequest(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor.LogActivity(CounterType counterType, String organizationName)
   at Microsoft.Exchange.Management.ControlPanel.RbacContext.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.get_Session()
   at Microsoft.Exchange.Management.ControlPanel.AuthenticationSettings..ctor(HttpContext context)
   at Microsoft.Exchange.Management.ControlPanel.RbacModule.Application_PostAuthenticateRequest(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

System.InvalidOperationException: Cannot load Counter Name data because an invalid index '' was read from the registry.
   at System.Diagnostics.PerformanceCounterLib.GetStringTable(Boolean isHelp)
   at System.Diagnostics.PerformanceCounterLib.get_NameTable()
   at System.Diagnostics.PerformanceCounterLib.get_CategoryTable()
   at System.Diagnostics.PerformanceCounterLib.CategoryExists(String machine, String category)
   at System.Diagnostics.PerformanceCounterCategory.Exists(String categoryName, String machineName)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor..cctor()
   at System.Diagnostics.PerformanceCounterLib.GetStringTable(Boolean isHelp)
   at System.Diagnostics.PerformanceCounterLib.get_NameTable()
   at System.Diagnostics.PerformanceCounterLib.get_CategoryTable()
   at System.Diagnostics.PerformanceCounterLib.CategoryExists(String machine, String category)
   at System.Diagnostics.PerformanceCounterCategory.Exists(String categoryName, String machineName)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor..cctor()

Flight info: Features:[[Global.DistributedKeyManagement, False],[Global.FrontdoorDefaultURL, False],[Global.GlobalCriminalCompliance, False],[Global.MultiTenancy, False],[Global.PopulateGroupMasterSid, False],[Global.WindowsLiveID, False],[Eac.AllowMailboxArchiveOnlyMigration, True],[Eac.AllowRemoteOnboardingMovesOnly, False],[Eac.AllowSender, False],[Eac.AntiSpamBulkThresholdUI, False],[Eac.AntispamTenantAllowBlockLists, False],[Eac.AtpPolicyForO365, False],[Eac.BulkPermissionAddRemove, True],[Eac.CaseHoldQuery, False],[Eac.CaseHoldSearch, False],[Eac.CmdletLogging, True],[Eac.ComplianceAllPublicFolderSearch, False],[Eac.ComplianceAllPublicFolderSearchForHold, False],[Eac.ComplianceCase, False],[Eac.ComplianceCaseClosing, False],[Eac.ComplianceCaseSources, False],[Eac.ComplianceExportIndividualMessageFormat, False],[Eac.ComplianceExportZipFormat, False],[Eac.ComplianceMnc, False],[Eac.ComplianceNewValidator, False],[Eac.ComplianceSearchRefiners, False],[Eac.ComplianceSearchStatistics, False],[Eac.ConditionCards, False],[Eac.ConditionCardsForCaseHoldQuery, False],[Eac.ConvertMailboxUI, False],[Eac.CrossPremiseMigration, False],[Eac.CustomizableMaxMsgSizeUI, True],[Eac.CutomizableSenderAuthenticationInNewDistributionGroup, False],[Eac.DefensibilityReport, False],[Eac.DeleteMessage, False],[Eac.DeleteQuarantineMessage, False],[Eac.DevicePolicyMgmtUI, False],[Eac.DiscontinueSafetenantConnectorUI, False],[Eac.DiscoveryDocIdHint, False],[Eac.DiscoveryMultiHoldSearch, False],[Eac.DiscoveryMultiQuerySearch, False],[Eac.DiscoveryMultiQueryV2Export, False],[Eac.DiscoveryPFSearch, True],[Eac.DiscoverySearchStats, False],[Eac.DiscoveryV1Export, True],[Eac.DistributionToUnifiedGroupMigrationEac, False],[Eac.DKIMSigningConfigUI, False],[Eac.DLPBlockForUnifiedCompliance, False],[Eac.DlpFingerprint, False],[Eac.DlpPolicyDetailsAndError, False],[Eac.DLPUnifiedCompliancePreview, False],[Eac.DLPWarnForUnifiedComplianceAvailability, False],[Eac.EACClientAccessRulesEnabled, True],[Eac.EacConciergeControl, False],[Eac.EacFeedbackControl, False],[Eac.EacPswsProxy, False],[Eac.EDiscoveryEacDecommisionExemption, True],[Eac.EdiscoveryExportPaging, False],[Eac.EdiscoveryExportV2AnalyzeWithZoom, False],[Eac.EdiscoveryExportV2ClientExportToolLiteEngine, False],[Eac.EdiscoveryExportV2General, False],[Eac.EdiscoveryHold, False],[Eac.EdiscoveryPreviewSearchUI, True],[Eac.EdiscoverySearchUI, False],[Eac.EDiscoverySPOMultiGEOEnabled, False],[Eac.EnableForInternalSendersInUI, False],[Eac.ExportDedupe, False],[Eac.ExportReportOnly, False],[Eac.FileFilter, False],[Eac.GeminiShell, False],[Eac.GrantSendOnBehalfToForSharedMailbox, False],[Eac.GroupsBulkUpgradeWizard, False],[Eac.GroupsInOutlookPromotionBanner, False],[Eac.HoldForModernGroups, False],[Eac.InactiveMailboxForCaseHold, False],[Eac.InactiveMailboxPickerEmailAddress, True],[Eac.InactiveMailboxSearch, False],[Eac.IsDedicatedTenant, False],[Eac.ManageMailboxAuditing, False],[Eac.ModernGroupCreateOnBehalfOf, False],[Eac.ModernGroupDelegatedUserSupport, False],[Eac.ModernGroupDomainSelectionSupport, False],[Eac.ModernGroupEnableDeliveryManagement, False],[Eac.ModernGroupManagement, False],[Eac.ModernGroupMoreEditOptions, False],[Eac.ModernGroupNonMailboxUserSupport, False],[Eac.ModernGroups, False],[Eac.ModernGroupsAllowAddingGuestInO365GroupMembership, False],[Eac.ModernGroupsAllowCreationByNonAADAdmin, False],[Eac.ModernGroupsPromotion, False],[Eac.ModernGroupsPromotionV2, False],[Eac.ModernGroupsSendOnBehalfOf, False],[Eac.ModernGroupsSkipVerifyOwnerLimit, True],[Eac.NewAuditingOptInUIOptIn, True],[Eac.NewAuditingReportUIOptIn, True],[Eac.NonExchangeWorkloadsUI, False],[Eac.Office365DIcon, False],[Eac.OrgIdADSeverSettings, False],[Eac.PreviewQuarantineMessage, False],[Eac.PreviewQuarantineMessageAdvanced, False],[Eac.PreviewSnapshotSearchUI, False],[Eac.PromoteProtectionCenter, False],[Eac.ProtectionCenterForceRedirect, False],[Eac.ProtectionCenterOptIn, False],[Eac.ProtectionCenterOptOut, False],[Eac.PublicFolderHoldManagement, False],[Eac.QuarantineMalware, False],[Eac.RemoteDomain, False],[Eac.RestoreUnifiedGroup, False],[Eac.RetentionPoliciesEacDecommisionExemption, True],[Eac.RetentionTagsEacDecommisionExemption, True],[Eac.RmsDecode, False],[Eac.SafeAttachments, False],[Eac.SafeAttachmentsDynamicDelivery, True],[Eac.SafelinkConvergedView, True],[Eac.SafeLinks, False],[Eac.SafeLinksBlockListFlexibleUrlPattern, False],[Eac.SafeLinksDoNotRewriteUrlByFlexibleUrlPattern, False],[Eac.SafeLinksScanUrls, True],[Eac.SelectivelyExportItemsById, False],[Eac.SetAtpPolicyForO365CmdletWACOption, False],[Eac.ShowExternalStorageWarningInGCC, False],[Eac.SingleFolderExport, False],[Eac.SPOPickerSearchAllTenantContent, False],[Eac.SupervisoryReview, False],[Eac.SupportAdfsIdentityInEcpProxy, False],[Eac.TestTransferToE164Extension, False],[Eac.ToggleABQWarning, False],[Eac.UCCAlertsReportingUI, False],[Eac.UCCAuditReports, False],[Eac.UCCIngestionUI, False],[Eac.UCCPermissions, False],[Eac.UCCTestProbeUI, False],[Eac.UnifiedAuditPolicy, False],[Eac.UnifiedAuditReportUI, False],[Eac.UnifiedComplianceCenter, False],[Eac.UnifiedDlpGA, False],[Eac.UnifiedPolicy, True],[Eac.UnifiedRetention, False],[Eac.UnlistedServices, True],[Eac.UseDoNotRewriteUrlsParamInSafeLinksPolicyCmdletAndUI, False],[Eac.WorkloadUIInUrlTraceTab, False],],  Flights:[],  Constraints:[[MACHINE, EXCHANGE],[MODE, ENTERPRISE],[PROCESS, MSEXCHANGEECPAPPPOOL],], IsGlobalSnapshot: True

Followed by:
Current user: 'FR8\HealthMailbox35665c6'
Request for URL 'https://localhost:444/ecp/About.aspx' failed with the following error:
System.TypeInitializationException: The type initializer for 'Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor' threw an exception. ---> System.InvalidOperationException: Cannot load Counter Name data because an invalid index '' was read from the registry.
   at System.Diagnostics.PerformanceCounterLib.GetStringTable(Boolean isHelp)
   at System.Diagnostics.PerformanceCounterLib.get_NameTable()
   at System.Diagnostics.PerformanceCounterLib.get_CategoryTable()
   at System.Diagnostics.PerformanceCounterLib.CategoryExists(String machine, String category)
   at System.Diagnostics.PerformanceCounterCategory.Exists(String categoryName, String machineName)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor.LogActivity(CounterType counterType, String organizationName)
   at Microsoft.Exchange.Management.ControlPanel.RbacContext.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.get_Session()
   at Microsoft.Exchange.Management.ControlPanel.AuthenticationSettings..ctor(HttpContext context)
   at Microsoft.Exchange.Management.ControlPanel.RbacModule.Application_PostAuthenticateRequest(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor.LogActivity(CounterType counterType, String organizationName)
   at Microsoft.Exchange.Management.ControlPanel.RbacContext.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.CreateSession()
   at Microsoft.Exchange.Management.ControlPanel.RbacSettings.get_Session()
   at Microsoft.Exchange.Management.ControlPanel.AuthenticationSettings..ctor(HttpContext context)
   at Microsoft.Exchange.Management.ControlPanel.RbacModule.Application_PostAuthenticateRequest(Object sender, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

System.InvalidOperationException: Cannot load Counter Name data because an invalid index '' was read from the registry.
   at System.Diagnostics.PerformanceCounterLib.GetStringTable(Boolean isHelp)
   at System.Diagnostics.PerformanceCounterLib.get_NameTable()
   at System.Diagnostics.PerformanceCounterLib.get_CategoryTable()
   at System.Diagnostics.PerformanceCounterLib.CategoryExists(String machine, String category)
   at System.Diagnostics.PerformanceCounterCategory.Exists(String categoryName, String machineName)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor..cctor()
   at System.Diagnostics.PerformanceCounterLib.GetStringTable(Boolean isHelp)
   at System.Diagnostics.PerformanceCounterLib.get_NameTable()
   at System.Diagnostics.PerformanceCounterLib.get_CategoryTable()
   at System.Diagnostics.PerformanceCounterLib.CategoryExists(String machine, String category)
   at System.Diagnostics.PerformanceCounterCategory.Exists(String categoryName, String machineName)
   at Microsoft.Exchange.Configuration.TenantMonitoring.TenantMonitor..cctor()

Flight info: Features:[[Global.DistributedKeyManagement, False],[Global.FrontdoorDefaultURL, False],[Global.GlobalCriminalCompliance, False],[Global.MultiTenancy, False],[Global.PopulateGroupMasterSid, False],[Global.WindowsLiveID, False],[Eac.AllowMailboxArchiveOnlyMigration, True],[Eac.AllowRemoteOnboardingMovesOnly, False],[Eac.AllowSender, False],[Eac.AntiSpamBulkThresholdUI, False],[Eac.AntispamTenantAllowBlockLists, False],[Eac.AtpPolicyForO365, False],[Eac.BulkPermissionAddRemove, True],[Eac.CaseHoldQuery, False],[Eac.CaseHoldSearch, False],[Eac.CmdletLogging, True],[Eac.ComplianceAllPublicFolderSearch, False],[Eac.ComplianceAllPublicFolderSearchForHold, False],[Eac.ComplianceCase, False],[Eac.ComplianceCaseClosing, False],[Eac.ComplianceCaseSources, False],[Eac.ComplianceExportIndividualMessageFormat, False],[Eac.ComplianceExportZipFormat, False],[Eac.ComplianceMnc, False],[Eac.ComplianceNewValidator, False],[Eac.ComplianceSearchRefiners, False],[Eac.ComplianceSearchStatistics, False],[Eac.ConditionCards, False],[Eac.ConditionCardsForCaseHoldQuery, False],[Eac.ConvertMailboxUI, False],[Eac.CrossPremiseMigration, False],[Eac.CustomizableMaxMsgSizeUI, True],[Eac.CutomizableSenderAuthenticationInNewDistributionGroup, False],[Eac.DefensibilityReport, False],[Eac.DeleteMessage, False],[Eac.DeleteQuarantineMessage, False],[Eac.DevicePolicyMgmtUI, False],[Eac.DiscontinueSafetenantConnectorUI, False],[Eac.DiscoveryDocIdHint, False],[Eac.DiscoveryMultiHoldSearch, False],[Eac.DiscoveryMultiQuerySearch, False],[Eac.DiscoveryMultiQueryV2Export, False],[Eac.DiscoveryPFSearch, True],[Eac.DiscoverySearchStats, False],[Eac.DiscoveryV1Export, True],[Eac.DistributionToUnifiedGroupMigrationEac, False],[Eac.DKIMSigningConfigUI, False],[Eac.DLPBlockForUnifiedCompliance, False],[Eac.DlpFingerprint, False],[Eac.DlpPolicyDetailsAndError, False],[Eac.DLPUnifiedCompliancePreview, False],[Eac.DLPWarnForUnifiedComplianceAvailability, False],[Eac.EACClientAccessRulesEnabled, True],[Eac.EacConciergeControl, False],[Eac.EacFeedbackControl, False],[Eac.EacPswsProxy, False],[Eac.EDiscoveryEacDecommisionExemption, True],[Eac.EdiscoveryExportPaging, False],[Eac.EdiscoveryExportV2AnalyzeWithZoom, False],[Eac.EdiscoveryExportV2ClientExportToolLiteEngine, False],[Eac.EdiscoveryExportV2General, False],[Eac.EdiscoveryHold, False],[Eac.EdiscoveryPreviewSearchUI, True],[Eac.EdiscoverySearchUI, False],[Eac.EDiscoverySPOMultiGEOEnabled, False],[Eac.EnableForInternalSendersInUI, False],[Eac.ExportDedupe, False],[Eac.ExportReportOnly, False],[Eac.FileFilter, False],[Eac.GeminiShell, False],[Eac.GrantSendOnBehalfToForSharedMailbox, False],[Eac.GroupsBulkUpgradeWizard, False],[Eac.GroupsInOutlookPromotionBanner, False],[Eac.HoldForModernGroups, False],[Eac.InactiveMailboxForCaseHold, False],[Eac.InactiveMailboxPickerEmailAddress, True],[Eac.InactiveMailboxSearch, False],[Eac.IsDedicatedTenant, False],[Eac.ManageMailboxAuditing, False],[Eac.ModernGroupCreateOnBehalfOf, False],[Eac.ModernGroupDelegatedUserSupport, False],[Eac.ModernGroupDomainSelectionSupport, False],[Eac.ModernGroupEnableDeliveryManagement, False],[Eac.ModernGroupManagement, False],[Eac.ModernGroupMoreEditOptions, False],[Eac.ModernGroupNonMailboxUserSupport, False],[Eac.ModernGroups, False],[Eac.ModernGroupsAllowAddingGuestInO365GroupMembership, False],[Eac.ModernGroupsAllowCreationByNonAADAdmin, False],[Eac.ModernGroupsPromotion, False],[Eac.ModernGroupsPromotionV2, False],[Eac.ModernGroupsSendOnBehalfOf, False],[Eac.ModernGroupsSkipVerifyOwnerLimit, True],[Eac.NewAuditingOptInUIOptIn, True],[Eac.NewAuditingReportUIOptIn, True],[Eac.NonExchangeWorkloadsUI, False],[Eac.Office365DIcon, False],[Eac.OrgIdADSeverSettings, False],[Eac.PreviewQuarantineMessage, False],[Eac.PreviewQuarantineMessageAdvanced, False],[Eac.PreviewSnapshotSearchUI, False],[Eac.PromoteProtectionCenter, False],[Eac.ProtectionCenterForceRedirect, False],[Eac.ProtectionCenterOptIn, False],[Eac.ProtectionCenterOptOut, False],[Eac.PublicFolderHoldManagement, False],[Eac.QuarantineMalware, False],[Eac.RemoteDomain, False],[Eac.RestoreUnifiedGroup, False],[Eac.RetentionPoliciesEacDecommisionExemption, True],[Eac.RetentionTagsEacDecommisionExemption, True],[Eac.RmsDecode, False],[Eac.SafeAttachments, False],[Eac.SafeAttachmentsDynamicDelivery, True],[Eac.SafelinkConvergedView, True],[Eac.SafeLinks, False],[Eac.SafeLinksBlockListFlexibleUrlPattern, False],[Eac.SafeLinksDoNotRewriteUrlByFlexibleUrlPattern, False],[Eac.SafeLinksScanUrls, True],[Eac.SelectivelyExportItemsById, False],[Eac.SetAtpPolicyForO365CmdletWACOption, False],[Eac.ShowExternalStorageWarningInGCC, False],[Eac.SingleFolderExport, False],[Eac.SPOPickerSearchAllTenantContent, False],[Eac.SupervisoryReview, False],[Eac.SupportAdfsIdentityInEcpProxy, False],[Eac.TestTransferToE164Extension, False],[Eac.ToggleABQWarning, False],[Eac.UCCAlertsReportingUI, False],[Eac.UCCAuditReports, False],[Eac.UCCIngestionUI, False],[Eac.UCCPermissions, False],[Eac.UCCTestProbeUI, False],[Eac.UnifiedAuditPolicy, False],[Eac.UnifiedAuditReportUI, False],[Eac.UnifiedComplianceCenter, False],[Eac.UnifiedDlpGA, False],[Eac.UnifiedPolicy, True],[Eac.UnifiedRetention, False],[Eac.UnlistedServices, True],[Eac.UseDoNotRewriteUrlsParamInSafeLinksPolicyCmdletAndUI, False],[Eac.WorkloadUIInUrlTraceTab, False],],  Flights:[],  Constraints:[[MACHINE, EXCHANGE],[MODE, ENTERPRISE],[PROCESS, MSEXCHANGEECPAPPPOOL],], IsGlobalSnapshot: True

can you use an admin cmd box:

LODCTR /r

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I should note that before that, I'd ecreated the ECP directory with the link Kimputer suggested.  But it still wasn't working until MAS recycled the app pools.  All good now, thanks!