• In viewing or grading the assignment of students, the progress wheel just keeps spinning and I cannot see the submission.

Solution: press “Change user” option (upper right) and reload. See illustration below. (The viewer could just be stuck with a document for which Moodle doesn't have the right viewer. Pdf files should work just fine. Your browser should also display such document type.) Cycle through such “Change user” option, especially if initially the pdf document doesn't load immediately.