Comment # 11 on bug 966671 from Michal Kube��ek

I've seen this problem twice so far, once yesterday and once today, on 42.2
(upgraded from 13.1 ~2 weeks ago). Running xauth under strace told me that
missing /tmp/xauth-1000-_0 is the problem (1000 is my uid, "_0" is probably
$DISPLAY with colon replaced by an underscore). Once I did

  cp ~/.Xauthority /tmp/xauth-1000-_0

everything started to work again.

Then I tried to disable systemd-tmpfiles-clean.timer (suggested by dsterba),
let's see if this prevents the problem from reappearing (I didn't end the
KDE session).

One theory why this only started to happen yesterday would be that I switched
from KDM to SDDM earlier this week (probably on Tuesday).