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:
So, finally we get rid of an bug introduced in Rollup 3 for Service Pack 2
Following (cosmetic) error in event-Log showed up on each Client Access Server with Event-ID 1 –> “Unhandled Exception “User setting ‘PreferredSite’ is not available.”
The RU3 Error: