Finally I decided to upgrade my system to openSUSE 12.1 from 11.4.
I updated the repositories urls, did zypper ref, zypper dup, zypper verify

After the upgrade process was done, I rebooted my system and got my opensuse 12.1 running,
BUT now, when I'm trying to login to kde using kdm, the screen blanks for a second and then I get kdm_greeter login screen again.

1. I tried to use xdm as login screen (changed DISPLAYMANAGER in /etc/sysconfig/displaymanager) and got the same result.
2. I tried choosing failsafe as a window manager, this one brings a xterm and when I run startkde I get my desktop back but this is not a good solution, maybe a fair workaround.

where should I look for a hint ??
couldn't find nothing in /var/log/messages or /var/log/Xorg.0.log