[support] Blank page of death after permission change

[Walter Garcia-Fontes]

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. 

-- 
Walter Garcia-Fontes
Barcelona