Some days ago one of my users reported 507 error messages during a Moodle quiz, so I created a forum thread here https://moodle.org/mod/forum/discuss.php?d=429146&parent=1727004
unfortunately I got no response, so now I want to share my findings in case you notice a similar situation.
My Moodle (version 3.2.9) site is being runned in a Linux VPS with 16 of RAM and plenty SSD storage and CPanel software installed so it was strange to have an error message related with "Insufficient Storage".
The first finding is that memory_limit variable was set to 128M so I am raised its value to 512M
However, as that change didn´t fix the problem, my next findings were related to the Cloudlinux software that we use to provide a secure version of php 5.6 that is a requirement of my old Moodle version.
It seemed like its LVE limits were quite low and were causing some faults as you can see in the following pictures.
https://forums.cpanel.net/attachments/cloudlinux-adp-2-png.74545/
https://forums.cpanel.net/attachments/cloudlinux-adp-3-png.74549/
https://forums.cpanel.net/attachments/cloudlinux-adp-4-png.74553/
so the obvious solution was to raise those settings according to Cloudlinux documentation
https://support.cpanel.net/hc/en-us/articles/360052233674-How-do-I-adjust-limits-for-my-users-
In summary a "507 error" message not always means a storage error but it also be caused by an Input/Output Usage fault in Cloudlinux caused by simultaneous access of several users to a particular module that in my case was the quiz module.
So my final thoughts are if there is a different way to avoid those high Input/Output operations. Maybe some MySQL cache layer like REDIS?