The following error is seen in the MTA log:
CPermissions::SplitACL Warning 4522 Cannot apply an ACE object for...
This error is detailed in SOL13430:
Warning 4522 cannot apply an ACE for object found in mail or Public Folder target agent log file.
Howeverthe target objects need to be disabled because source credentials are being used to access the target mailbox & Associated External Account (AEA) permissions require the target object to remain disabled.
Permissions should be applied correctly to the target mailbox even if it is disabled, provided that the AEA permissions are set correctly on the target mailbox being synchronized & also the target objects which need to be given permissions to this mailbox.
If any of these disabled target objects do not have AEA permissions set, or they are applied to the SELF user instead of the source account, then the above error will appear.
Ensure that AEA permissions are set correctly for all disabled target mailboxes involved in the migration.