One theory would be that you have inadvertently damaged some part of the notebook infrastructure or metadata that the grader depends on. A way to test that theory would be to follow this thread to get a clean copy of the notebook. Then very carefully “copy/paste” over your completed code only touching the “YOUR CODE HERE” sections.
Please give that method a try and let us know if it helps or not.
One way to do that kind of damage is to run the notebook on a platform other than the course website and then upload it back to the course website.