[support] Blank page of death after permission change

I have found out recently that on the admin page for administering permissions after changing any permission I get a "blank page of death". The permission is changed anyway, and everything else continues working normally after hitting the "back" key or opening any part of my site.

I'm using 5.6 (planning to update to 5.7 anytime soon, but haven't found the time to do it).

How would I go diagnosing this? Which logs are useful to check, or anything else that would help me diagnosing a "blank page of death"? It's sort of frustrating to not get any type of error message in this type of situations.