The Task backup_cleanup_task wipes all temporary backup files older than 1 week (hard coded time). This means no matter what backup controllers are still left and if adhoc tasks still are queued for them, the files will be removed so the backup controllers will fail (see MDL-81511).

The default value for backup controllers to be removed is 30 days. So if your backup controller is between 7 und 30 days old, the cleanup task will wipe the corresponding files needed for restore.

Also, the files will be wiped in all cases, the backup controller will only be cleaned up, if there is no related adhoc task left.

Todo: The times of the file cleanup and the backup controller cleanup need to be synced.