by Przemek Kaszubski.
Hello,
I am experiencing the same problem - on an upgraded site. To my understanding this is because there are legacy files in upgraded courses, which - unfortunately - get backed up all en masse with any backups made in such a course. The solution that works, if I remember correctly, is to re-visit all the resources and activities that may be relying on legacy files and to refresh / recreate them in the settings by assigning files using the file picker, and then to delete the legacy files.