Comment # 35 on bug 859711 from Wolfgang Bauer

(In reply to James Knott from comment #34)
> I tried turning off IPv6 and it didn't make any difference.
Pity. I created packages with that patch from the RedHat bug report that indeed
fix my "remote" login problem on localhost (yes, I am aware that you don't try
to connect to localhost, but still). I would have hoped that they would help
you too, but if disabling IPv6 doesn't, they can't really either.

> Also, I have
> been running IPv6 on my network for almost 5 years and it was only after
> 11.4 that the problem occurred.
Yes, but a bug might have been introduced in kdm that broke its IPv6 handling.
If you dismiss that as possible problem because it worked 5 years ago, we can
just as well close the bug as you had no problem at all 5 years ago...

> One thing I noticed before and verified
> again today is that a new user will be able to connect for a while, before
> it fails for that user too.
Hm, would sound like the ~/.Xauthority file gets corrupted over time I'd say.
Something like this could also happen if the remote host's hostname or
IP-address changes while Xorg/kdm is running, I suppose.

Does it work again (for a while at least) if you remove ~/.Xauthority?
Does it work again immediately after a reboot (of the remote host)?

Also, can you please try to login to IceWM? And please try with xdm as
displaymanager on the remote system as well. (ideally all combinations: kdm and
IceWM, xdm and KDE, xdm and IceWM, you can set the default session for xdm in
/etc/sysconfig/windowmanager as it doesn't allow you to choose)
For me it is still not clear whether your problem is related to kdm or KDE in
any way or not.

Btw, to get xdm to accept XDMCP connections, you need to edit
/etc/X11/xdm/xdm-config as I found out today:
Just change the last line from "DisplayManager.requestPort:    0" to
"!DisplayManager.requestPort:    0", i.e. prepend a '!'.