During a migration using Migration Assistant my Synology DS220+ hang up because of a power outage. The Migration Assistant claimed that the source was already in migration. A reddit thread pointed towards the Migration Assistant Task Cleaner. But according to the logs it unfortunately got deleted. Since reinstalling the Migration Assistant didn't bring back the task I went via SSH into the file system and located the file that gets called by the task.
I executed the following script using python:
python /usr/syno/etc/synomigration/scripts/python/space/resource_cleaner.py
From that point on I could run the migration again. No more complaints.