I was doing a migration today for a client and whilst using the Exchange Management Console on Exchange 2007 to move their mailboxes, I got the following error for 9 of the users mailboxes.
Error was found for Username(user mail address) because: Error occurred in the step: Opening source mailbox. Failed to open mailbox with error: ClassFactory cannot supply requested class, error code: -1056749262
A little investigation showed that this was due to the users account being disabled. I used the Active Directory Users and Computers MMC to enable the users accounts. You then need to wait around 30minutes for AD replication to take place, and you MIGHT also need to restart the Information Store on the source server.
Further issues were found with some other users that had their mailbox permissions totally messed up. These had to be manually fixed before they could be relocated to the new server.