Are you getting this error when running Get-MailboxFolderPermission?
PS E:\> Get-MailboxFolderPermission deptcalendar@company.com:\calendar The security principal specified is already on the permission set. + CategoryInfo : NotSpecified: (:) [Get-MailboxFolderPermission], CorruptDataException + FullyQualifiedErrorId : [Server=ZN1UR12MB0288,RequestId=b4f155be-a238-4a98-8b3f-f3d9258e22d2,TimeStamp=6/19/2018 10:10:41 PM] [FailureCategory=Cmdlet-CorruptDataException] F8D77EE2,Microsoft.Exchange.Management.StoreTasks.GetMailboxFolderPermission + PSComputerName : outlook.office365.com
The error message seems cryptic, right? But it is not. It means there is a DUPLICATE entry for the same user (security principal) is in the permission set for the calendar folder permissions.
So how do we fix it?
The ONLY way is to use Outlook to remove the duplicate entry.
Yes, that means
- give yourself full access permission to the mailbox,
- create an Outlook profile (or just a calendar),
- go to Calendar, right click and go to properties, & then Permissions
- remove the duplicate user entry
Enjoy. 🙂
Anand, the Architect 
Outlook just said “Some Permissions cannot be displayed”, but then I found your other article https://anandthearchitect.com/2020/11/05/exchange-online-calendar-permissions-some-permissions-cannot-be-displayed/ and I applied that first… then this article worked. At first it didn’t work, I had to wait a while. I think it was synching outlook. Then it worked.. Thanks. You may want to link to your other article first at the top of this article.