 - What clients were running at the time the issue occurred
Skype, System Monitor, Firefox, Mate Terminal, Libre Office, Caja, TigerVNC,
Virtualbox, Pluma
 - Any special x11 client that can reproduce?
have no idea
 - Any particular action that triggers the issue?
Usually crashes during the night, when a black screen saver is running, but it
happened several times during the day, when I worked
 - which desktop environment/window manager?
LightDM/MATE

Basically, anything that could help reproducing...
Just leave X running long enough time - usually after 2-3 days without restart