Cannot proceed with the current migration session because the target mailbox is locked by *******. Another migration process is taking place in parallel.igration process is taking place in parallel. (4254433)

Cannot proceed with the current migration session because the target mailbox is locked by *******. Another migration process is taking place in parallel.igration process is taking place in parallel.

Cannot proceed with the current migration session because the target mailbox is locked by *******.  Another migration process is taking place in parallel.
 

The mailbox is locked in another task that is currently running. The migration task holding the lock could be an old migration task that failed or otherwise didn't finish in a clean way. And so that old task never released the lock. 

To avoid this error, ensure that the mailboxes are only added to a single running task.  The first task that is run will lock the user(s) mailbox to that task so that another mailbox migration cannot be run simultaneously.

If there are no other tasks running that are migrating that mailbox, then that indicates the mailbox is locked by some previous failed task. If so then log a Service request with Support and reference this Knowledge article. Support will find the offending migration task id and have that task cleaned up on the back end to release the lock..