You received the error, but was the move successful? During the move, It sounds like this user’s Outlook may have tried to log onto their mailbox while you were moving it. Exchange 2007 will not allow a logon to the Mailbox (for prevention of corruption). The User’s Outlook may have been trying to access the mailbox while it was being moved, and it was unable to. You can safely ignore this message during a Mailbox move.