Exchange 2010 has a feature where mailboxes can be quarantined, if they cause the store to crash. Quarantined Mailboxes are not accesssible. When a mailbox is quarantined, an Event ID:10018 will be logged by exchange to the Application Event log.
The mailbox will automatically un-quarantine itself after 6 hours. It can also be done manually by deleting a registry key and restarting the store. If a mailbox is quarantined repeatedly, Microsoft should be contacted to determine the cause of the store crash.
For Migration Manager 8.7 and earlier, the store issue will need to be resolved, as undoing the switch requires access to the target mailbox.