User mailbox calendar permissions issue

Hello,

We have a hybrid on-site/MS365 single tenant Exchange setup.

Currently I'm running into issues with a couple of mailboxes that have permissions set on them.

Behaviour:

When the user attempts to view calendar folder permissions from the Outlook 365 desktop app, the hourglass spins for awhile before displaying a dialog that says "Cannot display certain permissions". They can only click on Ok and then the Access Permissions tab is empty. If the user logs on to outlook.office365.com they can go to their calendar permissions and make changes to permissions. If I delete the user's Outlook profile on their workstation, and then close Outlook, reopen Outlook and re-create the profile, they can go view and/or make changes to calendar permissions once. Once they've opened that tab once, they cannot go back to it unless their profile is again deleted/recreated.

Investigation:

In Powershell, I was able to run Get-MailboxFolderPermission "<user>:\Calendar" to view permissions on that folder. There were a bunch of accounts there that no longer exist, and that were not visible in OWA. I then ran Remove-MailboxFolderPermission "<user>:\Calendar" -User <otheruser>  for each of the undesirable users. In some cases, I had to use the Full name in quotes ("Lastname, Firstname") for the -user switch as using the account, alias or email wasn't working, and for two accounts, which still exist in our GAL I was unable to remove them at all.

One thing that remains: The list of users with permissions to the calendar folder is different in Powershell than in Outlook, even after forcing sync between our on-prem Exchange host and O365.

None of the user's other folders are affected by the issue.

I've been researching this one for days now and can't find any references to this type of issue anywhere.

Any help much appreciated ! 

Felix