I have 1 user that keeps getting issues with his calendar where android gives a ! and 'the exchange server returned an error'
After digging around the logs i found an error with his name that states 'Invalid SyncKey' and after that it keeps going [CP WSGIServer Thread-22] [sync:1057] [userID:cn=user,ou=context] [eventID:] [objectID:] [Sync] Event has already been processed.

When i re-init the user or the device it'll be fixed for a few days but the problem keeps popping back up for him.

Any ideas on what might be causing this ?