Take Ownership of the Exercises
This link allows the teacher to take ownership of the current set of exercise submissions. You will need to do this if you have imported an exercise from a backup of another course. This link allows you to "claim" all the exercise submissions. In the current course these imported submissions could belong to anyone, even a student. This procedure avoids any problems that this may cause and makes you their owner.
Apart from onwership, the submissions themselves are unchanged by this procedure.