Microsoft Outlook has stopped working with new appointment in delegated calendar

"Microsoft Outlook has stopped working" message appears when opening up a new calendar appointment on a mailbox where there are 'Editor' delegated permissions. Open an existing appointment will not cause this issue.  

Outlook will close with message:
  Problem signature:
  Problem Event Name:      APPCRASH
  Application Name:      OUTLOOK.EXE
  Application Version:      15.0.4859.1000
  Application Timestamp:      57b2d3f7
  Fault Module Name:      olmapi32.dll
  Fault Module Version:      15.0.4859.1000
  Fault Module Timestamp:      57b2d1d1
  Exception Code:      c0000005
  Exception Offset:      0000000000080af0
  OS Version:      6.1.7601.2.1.0.256.48
  Locale ID:      1033

There are various users that have access to this calendar, some of them have this issue, some do not.  This issue can be replicated on different computers and using different OS’s and Outlook versions (2010, 2013 & 2016).  This issue doesn’t occur when using OWA nor when using Outlook 2016 mac … only windows.  It also occurs when starting Outlook in safe mode.  All desktop OS’s are fully patched (Windows 7/10) and all Outlook clients have latest patches installed.

All users were working fine for this shared mailbox up until about a week or so ago.  This issue can be replicated at will with users that exhibit this issue, regardless of computer/outlook used. Access to other shared mailboxes do not exhibit this issue from same user.

We are not aware of any change that occurred on our systems that correlate with this issue.  Our exchange is on-premises 2013 CU12.  Servers are running Windows 2012 R2, with fully patched OS’s. Most OS and application updates occurred mid-September.  Outlook logging and Event viewer do not reveal any more that Outlook had an application crash with event ID’s 1000 & 1001.  We have tried creating a new profile and reinstalling Office without a resolution.

At this point it seems to be only a single shared mailbox that this issue occurs on.  Removing the user as delegate and adding them again doesn’t change the issue. The recipient type details for the shared mailbox is “UserMailbox”. The mailbox owner doesn’t have any issues creating a new appointment, just some of his delegates.  For the users that can create new appointments; we have not tried to remove and create delegation for fear that they may break to.  We have run server based mailbox repair on delegator’s mailbox (New-MailboxRepairRequest all CorruptionType) without resolution.

Here is a sanitized version of mailbox permissions

Get-MailboxFolderPermission -Identity Boss@contoso.com:\Calendar
FolderName  User      AccessRights
----------  ----      ------------
Calendar    Default   {None}
Calendar    Anonymous {None}
Calendar    Steve     {Editor}
Calendar    Brad      {Editor}
Calendar    Amy       {Editor}
Calendar    Erica     {Editor}
Calendar    Mary      {Editor}
Calendar    Kris      {Editor}
Calendar    Ely       {Editor}
Steve Brad and Amy have this issue Erica Mary Kris and Ely do not.  Any new person who has access added to this calendar has this issue.

Logic dictates there is an issue with the owners calendar but why is only occurring for some delegators?  Is anyone aware of any changes occurring in the past couple weeks that might explain this odd behavior only with Outlook for Windows email clients?

We are also have an issue with a room mailbox were an editor who had access now doesn’t when using Outlook. It still works using OWA. While outlook doesn’t crash it does give the a message that this user doesn’t have permissions.  I don't know if this other issue is coincidental or not.
winsystemsAsked:
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winsystemsAuthor Commented:
Case was opened with Microsoft and turned out to be an issue with delegator's Calendar mail folder.  Someone modified this to be other than IPM.Appointment when posting to this folder with custom form that wasn't valid for all delegates

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