Exchange 2013 Migration Batch Completes Successfully, But Mailbox is Not Moved

Here's another weird one. I'm testing a mailbox move from Exchange 2013 to Exchange 2016 (beta) for a mailbox with an in-place archive. I can create a new migration batch for the user and it completes in short order, but the mailbox and archive are not moved. The number of Total, Synced, Failed, and Successful mailboxes for the batch is zero. I get a notification email saying the batch completed successfully, but no mailboxes were moved or synced.

"Successful" Migration Batch Notification

When I look at the details of the migration batch in EMS I see that the ValidationWarningCount is 1 and the ValidationWarnings show, "Error:The user already exists, but the migration batch that includes it couldn't be found. Before you try migrating the user within a batch again, please remove the existing user by running the Remove-MigrationUser cmdlet."

Migration Batch Validation Warnings

Whoever wrote that error message deserves a raise. MUCH better than the "Error code 2" I'm used to getting.

A "MigrationUser" object is created for each user who is in the process of being migrated or the migration has not been completed, so it remains in a syncing state. I ran the Get-MigrationUser cmdlet which revealed that the mailbox was indeed "stuck" in a migration since 7/20/2013(!!) with no other issues.

This mailbox has been "migrating" for over two years!

So, I removed the migration user using the Remove-MigrationUser cmdlet. I needed to use the -Force parameter since the move request no longer exists.

Remove-MigrationUser clears the way to re-run the migration

This fixed the problem and I was able to move the mailbox and archive successfully. This condition would prevent a mailbox from moving to any other database or Office 365. It's strange that Exchange doesn't detect this when the batch request is created.