We receive the following error during ReACL: Unable to process User Profile S-1-5-21-XXXXXXX-11297XXX02-XX255446XX-XXXXX System.NullReferenceException: Object reference not set to an instance of an object.
Mapping files downloaded fine. Some failed profiles are not in the migration scope and some are. All failed.
Confirm user is in scope for the DirSync workflow, it exists in both source and target and is matched.
Confirm the user exists in the map.usr file and this file was downloaded onto the device during the ReACL.
The issue may also be caused by old profiles on the machine, that are no longer exist in the source domain as SIDs. Ideally, it's recommended to cleanup old profiles from the machine using Control Panel->System->Advanced System Settings->User Profiles->Settings. Check what profiles are no longer relevant and delete those.