When attempting to remove permissions from Exchange server mailbox using Exchange Management Shell… Powershell
You get the following error
WARNING: Can’t remove the access control entry on the object… because the ACE doesn’t exist on the object.
There is abundance of articles on the Internet on to deal with this… mostly confusing and convoluted.
The easiest way we’ve succeeded in resolving this issue is be removing the mailbox from the user and adding it again. That doesn’t mean you will delete anything..
In Exchange 2013 Admin Center if you go to that user under Recipient and then Mailboxes then disable the user (that will remove the mailbox from the user profile). You can then go back to and select “connect a mailbox” and connect that mailbox back to the same user account. This way you will have reset permissions on the mailbox to default. Issue was resolved for us this easy way.