The Google Form has a file upload question. Could a problem with that question be causing the submit button to be disabled?

Yes, file upload questions are a common culprit. The most frequent issue is that you haven't selected or uploaded a file where required. Secondly, the file type may be restricted and the file you're trying to upload might not be permitted. Confirm the file type requirements specified by the form creator. Also, ensure that your file size meets the upload size limit specified by the form creator. Lastly, if the file upload is taking an unusually long time, the form may appear frozen.