Validation and nb-grader problems

I’ve seen this error reported occasionally over the years. It always turns out to be that some part of the notebook has been damaged outside of the code areas. Your best bet is to use the topic on the FAQ Thread to get a clean copy of this notebook. Save yours first, of course, under a different name. Then very carefully “copy/paste” over just your completed code into the fresh copy and see if that helps. Let us know if that doesn’t work!