Server Error in /OMA

Some (not all) of our email users have problems with Outlook Mobile Access.  

Server Error in /OMA
System.NullReferenceException Object reference not set to an instance of an object. Method: GetUserDateFormat.
Stack trace:
At Microsoft.Exchange.Oma.Preferencing.OmaUserInfo.GetUserDateFormat(String userCultureName) at Microsoft.Exchange.OMA.Preferencing.OmaUserInfo.get_UserCulture() at Microsoft.Exchange.OMA.UserInterface.Page.Page_Load(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.MobileControls.Mobilepage.OnLoad(EventArgs e) at System.Web.UI.Control

When they try https://sub.domain.com/oma
they are prompted for username and password, then get:
A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator"A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator."

Two errors in Application Event Log:

Source: MSExchangeOMA
Category: (1000)
Event ID 1506
An unknown error occurred while processing the current request: Exception of type System.Web.HttpUnhandledException was thrown.
Stack trace:
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain()
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at System.Web.CallHandlerExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
 Inner Error: Object reference not set to an instance of an object.
 Stack trace:
   at Microsoft.Exchange.OMA.Preferencing.OmaUserInfo.GetUserDateFormat(String userCultureName)
   at Microsoft.Exchange.OMA.Preferencing.OmaUserInfo.get_UserCulture()
   at Microsoft.Exchange.OMA.UserInterface.Page.Page_Load(Object sender, EventArgs e)
   at System.Web.UI.Control.OnLoad(EventArgs e)
   at System.Web.UI.MobileControls.MobilePage.OnLoad(EventArgs e)
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain()
 
Source: MSExchangeOMA
Category: (1000)
Event ID 1507

An unknown error occurred while processing the current request: Error executing child request for oma.aspx.

 Stack trace:
   at System.Web.HttpServerUtility.ExecuteInternal(String path, TextWriter writer, Boolean preserveForm)
   at System.Web.HttpServerUtility.Transfer(String path, Boolean preserveForm)
   at Microsoft.Exchange.OMA.UserInterface.UserError.OkClick(Object sender, EventArgs e)
   at System.Web.UI.MobileControls.Command.OnClick(EventArgs e)
   at System.Web.UI.MobileControls.Command.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String argument)
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.MobileControls.MobilePage.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
   at System.Web.UI.Page.ProcessRequestMain()
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at System.Web.CallHandlerExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
 Inner Error: Exception of type System.Web.HttpUnhandledException was thrown.
 Stack trace:
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain()
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at System.Web.HttpServerUtility.ExecuteInternal(String path, TextWriter writer, Boolean preserveForm)

 Inner Error: Object reference not set to an instance of an object.
 Stack trace:
   at Microsoft.Exchange.OMA.Preferencing.OmaUserInfo.GetUserDateFormat(String userCultureName)
   at Microsoft.Exchange.OMA.Preferencing.OmaUserInfo.get_UserCulture()
   at Microsoft.Exchange.OMA.UserInterface.Page.Page_Load(Object sender, EventArgs e)
   at System.Web.UI.Control.OnLoad(EventArgs e)
   at System.Web.UI.MobileControls.MobilePage.OnLoad(EventArgs e)
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain()Start Free Trial