I have spent most of the past two days on this problem and give up. One of our users is trying to cancel a meeting request that he set (and it is recurring) and receives this error: Error: You don't have the permissions required to send messages from this mailbox.
I’ve tried to cancel this meeting by logging in as this user in OWA and receive the same error message.
The one problem that I did find is pretty weird – when I used PowerShell to view this user's rights to both his email account and calendar it didn't look like he was the owner of either, so I did make this change. This might have fixed it and I have to wait for it to propagate; but it might not have fixed it and I may have created a big mess.