This issue has been logged as a product defect (defect ID PT139013717) and resolved in the latest cumulative update for 8.13 which can be downloaded here: https://support.quest.com/migration-manager-for-exchange/8.13/download-new-releases
The below information was copied from the release notes for the 8.13 hotfix:
Defect ID: PT139013717
Issue description: Migration of a mailbox fails if there is a user-created folder named 'Files' at source.
Scope: Mailbox migration from any supported source to Microsoft Office 365 or Exchange 2016 with Cumulative Update 4.
Root-cause: MAgE was not able to handle folders of IPF.Files class properly.
Fix: The algorithm of duplicate avoidance has been enhanced to handle a user folder named 'Files' properly.
Defect ID: PT139013717
Issue description: Migration of a mailbox always fails for the described scope.
Scope: Mailbox migration from Microsoft Exchange 2016 with Cumulative Update 4 to Microsoft Office 365 or Exchange 2016 with Cumulative Update 4.
Root-cause: MAgE was not able to handle folders of IPF.Files class properly.
Fix: A system folder Files introduced for Exchange 2016 is now excluded from mail migration using WellKnownFoldersToSkip parameter.