Had the same issue from our end on an upgrade from 1.52-1.9. Only way to fix is to manually delete from the database. This is what I did (I have myphpadmin installed):
Note your course ID
- On homepage - Go to Site administartion - server>>database (myphpadmin opens)
- Click on your database (top left)
- Scroll down to mdl_grade_items
- View the mdl_grade_items by course ID
- Scroll down to your course ID
- Delete all databse rows (in the course ID you want) with categorytype=NULL and Itemname=Null.
This will clean out all remnant grades.