Error 500 in Apache is very general. It only says the server can not handle the request, for any number of reasons. In your case, since you are not changing anything in the server and this happens during intensive assignment uploads, it could mean two things: either the server is simply too weak or your the document converter in your machine is inefficient thereby loading the server unnecessarily.
Increase the https://docs.moodle.org/38/en/Debugging level to its maximum and get more information on what is happening inside.
Increase the https://docs.moodle.org/38/en/Debugging level to its maximum and get more information on what is happening inside.