We are using Moodle 3.1 and we are experiencing a major problem with grading assignments. Teachers are complaining that Moodle hangs when they click "Grading" on the assignment grading table. User session gets completely freezed and they are only able to use moodle again after closing the browser.
This is not something that happens every time (making it difficult to reproduce) but I was able to reproduce it a couple of times. We get the error described on https://tracker.moodle.org/browse/MDL-57259 but even after patching moodle 3.1 the error persists.
We also have the following error log (example): Cannot obtain session lock for sid: e7ek6iojlde7nsf6p36apubid2, referer: https://[moodle]/mod/assign/view.php?id=12646&rownum=0&action=grader&userid=13458
We can't disable unoconv now because the pdf annotations done so far get lost if we do so, so we keep telling our teachers that they should enable quick grading and not touch on grading buttons! (They keep saying: "I never had problems before with grading assignments.... why did it changed?")
If someone has this same problem and was able to solve it somehow please let me know how you did it.