Most likely that means one of two things:
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A problem on the grading servers. Please try submitting again and see if the problem persists. If so then the next theory is:
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The metadata or other structural information in your notebook has been corrupted. To remedy this, you need to get a clean copy of the notebook and then carefully “copy/paste” over just your solution code from your damaged copy. There is a topic on the DLS FAQ Thread about how to get a clean notebook and those instructions work for NLP as well. Let me know if you have trouble following that link.