We've been experiencing an unusual problem with grading assignments. When an instructor clicks the "Save and Show Next" button after adding comments and grading the 1st student's assignment, the button works, the grade is saved, and the next student is displayed. However, if they then click "Save and Show Next" again after grading the 2nd student's work, a "Saving changes" message with a spinning circle shows up and the program freezes (the circle keeps spinning indefinitely). I've attached a screenshot of what it looks like. At that point, they have to either close out the window or click the Back button in their browser, either of which will cause the instructor's comments and grade for that student to be lost.
They can then go back to the 2nd student's assignment, click Save and Show Next, and it will work, but the program will freeze again when they click Save and Show Next on the 3rd student's assignment. So the button only works for one student at a time and freezes on the next one.
As far as I can tell, the problem surfaced after we patched our Linux servers last month. We use Centos 7 with Webmin, and we updated the servers through the Webmin interface. However, I'm having trouble figuring out how that process could have caused this issue. Possibly an update to PHP? We're using PHP 7.2.24 running in FCGI, and Zend OpCache is enabled. The resource limits (max input time, etc.) are all set fairly high.
I also thought it could be the PDF annotation with unoconv and LibreOffice since those were likely also updated, but the same error happens even if PDF Annotation is disabled for the assignment.
We are using Moodle 3.5.9+ (Build: 20191122). I attempted to upgrade to 3.9 last month, but unfortunately it broke our theme (Adaptable) when we tried the upgrade on a test server. Hopefully we'll be able to do that after this term, but in the meantime, it would be really helpful for our faculty if we could figure out why the grading screen keeps freezing.
EDIT: I've found another glitch on the 2nd student's screen. The "Change Users" section in the top right doesn't work at all (left and right arrows do nothing and it won't change if you select a student from the dropdown). Also, the dropdown doesn't display the correct number of students even though no filters are applied. So somehow this 2nd student's grade screen is already glitching before the instructor clicks "Save and Show Next." In fact, they don't even have to click "Save and Show Next" on the 2nd student. Just clicking Save Changes will also freeze the screen.
I don't know if anyone else has run into a similar issue, but thanks in advance for any help or advice!