(In reply to Michel Dänzer from comment #1)
> Please attach the Xorg log and dmesg output corresponding to the problem.
> 
> (In reply to Bernd Steinhauser from comment #0)
> > * x11-server/xorg-server-1.18.4::x11 2016-07-20 16:06:18 UTC
> 
> Which version of xorg-server were you using before? Does going back to that
> fix the problem?

Before it was 1.18.3 installed in April. I hoped that the update might improve
the situation, but it didn't.
So I'm pretty sure that the xorg-server update is unrelated.