Sometimes delegate access for an Exchange Online calendar goes awry due to corrupted items in the mailbox. To help sort out problems, Microsoft has upgraded the Remove-MailboxFolderPermission cmdlet to do the work that used to be done by a multi-phase fix performed using the MFCMAPI or EWS editor utilities. The nice thing is that this method is quick, simple, and works well.
https://office365itpros.com/2021/01/20/rebuild-delegate-access-calendar-powershell/
