This is a result because Exchange did not recognized that a disconnected mailbox already has been reconnected to a AD User. This seems to be a bug in Exchange 2010 SP2 RU1 and should be fixed in RU3 .
As Workaround for immediate success run:
Clean-Mailboxdatabase DBNAMExxx
http://technet.microsoft.com/en-gb/library/bb124076%28EXCHG.141%29.aspx
We had an environment with 2 exchange servers and 3 AD servers have a desync due to improper demotion, and had this on ~ 50-60 accounts afterwards. Showed up from AD listing, but going to calendar tab elicited this error. Emails in would bounce with disabled account.
Command above worked flawlessly to repair the issue.