Comment # 8 on bug 1013200 from Patrick Schaaf

(In reply to Stefan Dirsch from comment #6)
> Hmm. The only difference in xorg-x11-server between 42.2 release and update
> I can find is
> 
>  U_modesetting-set-driverPrivate-to-Null-after-closing-fd.patch
>  Prevent crash when unplugging displaylink device. (bnc#1011570)
> 
> I don't think this is the culprit and it only affects modesetting driver
> users, which you probably are not with your AMD card (check
> /var/log/Xorg.0.log or wherever your displaymanager writes the X log into).

I'll attach my current Xorg.0.log, not sure exactly what to look for there.
There is a modesetting_drv loaded and notes about using "kms".

> > (from the "official" http://download.opensuse.org/repositories/Kernel:/stable/standard/ repository) 
> 
> For sure this is not the "official" kernel. This is a bleeding edge
> "kernel-of-the-day".

Yeah! But it's a nice one actually - never failed me for the last 6 months.
Anywway....

> Therefore I suggest to go back to our latest Leap 42.2
> kernel (version 4.4!) and test again.
> 
> For me this looks like a race between plymouth and X. Honestly if you can
> live
> without a splash screen for boot I recommend to disable plymouth by adding
> "plymouth.enable=0" to the kernel boot options. ;-)

Did both - back to the normal 42.2 kernel (kernel-default-4.4.27-2.1.x86_64),
the 4.8.x kernels removed. Tried with and without plymouth. NO CHANGE regarding
the issue, sddm-greeter even coredumps when plymouth is both disabled and
uninstalled (packages, initrd rebuild, though that I tested only with the
4.8.11 kernels)