Comment # 33 on bug 859711 from Wolfgang Bauer

Hm, this does indeed show a problem, but it's not very specific either.

So have you tried logging into IceWM? This should rule out a problem with the
(KDE) user config.

OTOH, by chance I stumbled over this RedHat bug report, which describes a
similar problem as you seem to have:
https://bugzilla.redhat.com/show_bug.cgi?id=1187957

There it was related to IPv6, in particular kdm apparently writes wrong entries
to .Xauthority for IPv6 loopback devices, causing the login session to not be
able to connect to the display.

So I'd suggest you try to disable IPv6 on the remote host, either in
YaST->Network Devices->Network Settings->Global Options (needs a reboot), or
temporarily with "sysctl -w net.ipv6.conf.all.disable_ipv6=1" (should have
immediate effect).
Does this help?

After reading that bugreport, I tried to "remote" connect to my localhost, and
indeed saw the same behaviour: the "remote" login screen appeared (I can tell
because the menu only contains "Close Connection" and "Shutdown"), but when I
try to login (even to IceWM) the session crashes immediately. Disabling IPv6
makes that work.

What I'd also try is to use a different displaymanager as a test. xdm should be
installed by default, and if you enabled XDMCP in YaST->Network
Services->Remote Administration or in /etc/sysconfig/displaymanager directly,
this should also apply to xdm.
To switch to xdm, set DISPLAYMANAGER="xdm" in /etc/sysconfig/displaymanager.