Exchange 2013: Cannot open mailbox cn=Microsoft System Attendant.

Cannot open mailbox /o=mydomain/ou=Exchange Administrative Group (H8I0BOHFHJ736)/cn=Configuration/cn=Servers/cn=MBX01/cn=Microsoft System Attendant.

This message appeared while attempting to create a room mailbox within ECP during a migration. To resolve this error we run the following to re-enable the migration arbitration mailbox:

Enable-Mailbox -Arbitration -Identity "Migration.8f3e7716-2011-43e4-96b1-aba62d229136"
Set-Mailbox "Migration.8f3e7716-2011-43e4-96b1-aba62d229136" -Arbitration –Management:$true