Never saw this before on Exchange 2010. Just moving empty mailboxes that where just created from one db to another. I am seeing a lot of these warnings in the application log. Move seems to go OK. In some case, the status says completed with warnings but the log show Request Completed Successfully.
Log Name: Application
Date: 6/13/2013 7:54:24 PM
Event ID: 9660
Task Category: Logons
User Display Name(/o=Company/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Display Name) failed to sign in because their mailbox is in the process of being moved.
I know for a fact that the user is NOT logged in or trying to login. It seems to happen with any mailbox. I did some checking on this and found it to be a common problem. It looks like it started with Exchange 2010 SP2 RU3 or 4 and apparently has carried into SP3. Another I noticed is that mailboxes now show up immediately in disconnected mailboxes after a move. I remember in the past, it took 24 hours or you had to do a clean-mailboxdatabase command against the database to get them to show up.
Using Exchange 2010 SP3 on Windows 2008 R2 SP1. Have two CAS servers and two MB servers in a two node DAG. Does anyone know if there is a fix for this. Apparently the move is none the worse but it takes longer to move a mailbox because it gets stuck at 99% complete and gets delayed when it tries to do it final cleanup.