According to the logs, one of the PDF conversion processes was running incorrectly, which caused memory problems, and because of this, one of the main system processes also stopped. 

We first tried to solve the problem at the site level using several methods.
In the first time that we turned off the use of converters in the "Manage document converters" menu, but its not solved the problem. 
In the next time we turn off the "Annotate PDF-assignfeedback_editpdf" plugin, in the plugins menu. Unfortunetly, this did not solve the problem either.

You can see in the running tasks that there is a stuck task that has been running for 8 hours. This happens several times and sometimes it runs in 10-12 hours, incorrectly. It generates a large DPF file, which consists of about 47,000 pages, thereby reducing memory usage.
Such tasks are constantly running, basically they do not cause problems, only this 1-1 is stuck.

We need some help how to debug this problem. When there is an exam, it is very unpleasant that you can stop and slow down because of this. Usually, up to 1,200-1,300 students take the exam at the same time.

I am attaching some more pictures of this problem

We found the pdf with many thousands of pages and lots of image files on this path:
/mnt2/moodle/moodledata/temp/assignfeedback_editpdf/pageimages/fd3a6d00ace83f0487d8dee1b255236b71dd5400/combined.pdf