If you have sufficient permissions, try running the following cmdlet to reset the delegate collection:
Remove-MailboxFolderPermission mailbox:\Calendar -ResetDelegateUserCollection
More information about the switch here: https://learn.microsoft.com/en-us/powershell/module/exchange/remove-mailboxfolderpermission?view=exchange-ps
The mailbox is in Office 365.
Yes, I can share in OWA.
I have many profiles in Outlook client that I use and this is the only one that I can't open the "Delegates Access" settings. Weird....
I'm also getting the same error message on a completely different computer with Outlook client. Outlook client 2019 BTW.