WORKAROUND
Apply 8.12 hotfix 20160804EX (or later)
STATUS
This is a confirmed defect fixed in public update 20160804EX (and inclusive in subsequent updates). For 8.12+ version customers the details of the public update are available at https://support.quest.com/migration-manager-for-exchange/kb/212197
Customers can keep availed of latest Migration Manager updates via https://support.quest.com/kb/77417