Transient error SourceMailboxAlreadyBeingMovedTransientException has occurred. The system will retry (5/620).This error occurs when a previous move attempt did not get cleaned up properly. From a bit of reading, this should timeout and fix itself after about 2 hours. However, since I didn't want to wait that long, I did the following that got it going again.
- IISReset.exe to restart the web services
- Restart the Microsoft Exchange Mailbox Replication service