Perhaps it's not a KDE bug, anyway... Start to Switch user -> start new session. Log in, you have now X servers on vt7 & vt8 and you can switch by ctrl-alt-f7/f8. Ok, now close new session started at vt8. This should terminate X :1. Now try to start another session again. The screen colors corrupted and everything freezes, reset hardware key only helps... Sergey.