I just tried submitting the Week 3 and Week 4 assignments from NLP Course 1 and they both worked for me. So this could be just a temporary server problem. Please try again and let us know. The most efficient way to submit is:
Kernel -> Restart and Clear Output
Save
Submit
That clears all the generated output from the notebook, so that the size of the file submitted is as small as possible. The grader does not need to see your generated output: it only needs to call your functions.