Comment # 9 on bug 854364 from Egbert Eich

(In reply to Axel Keller from comment #8)
> DISPLAYMANAGER=kdm
> 
> Sorry, that I didn't care about your question before.

Axel, thanks for confirming! kdm would be maintained by KDE. 
I believe I understand what is happening: Desktops like Gnome and KDE don't
allow multiple logins of the same user at the same time (I know for sure that
Gnome does this, not so sure about KDE), therefore when you have autologin
enabled you are logged in already. Trying to log in again as the same user from
remote will this be inhibited - thus the session is closed.

Therefore this more a feature than a bug.
You will get the same behaviour ssh-ed into the machine with X-forwarding and
did:
  Xephyr :1 -query localhost -retro

However, inspired by this I tested several DMs on Tumbleweed: sddm (the one KDE
uses to replace KDM), gdm and lightdm.

                   Autologin
                on         off
sddm            no XDMCP   no XDMCP
gdm             no XDMCP   no XDMCP
lightdm         XDMCP (*)  XDMCP(*)

(*) dm prevents 2nd login as same user - even for twm session, therefore, when
autologin is enabled, a remote login isn't possible (until local session is
logged out).

On 13.1 everything still works as expected (with some surprises) while on TW
neither sddm nor kdm allow XDMCP (at least not by setting
DISPLAYMANAGER_REMOTE_ACCESS="yes" in /etc/sysconfig/displaymanager).

Since the observed behaviour is actually intended, I'd like to close this as
WORKSFORME.
The 'no second login for the same user policy should be mentioned in the
release notes, though).