I don't even know where to start googling for this. As of this morning, one of my PCs, running Debian 3.1 unstable, KDM, KDE 3.5, fully updated, does this:
Boots to the usual login screen. I enter the password and it is as if I have pressed Ctrl-Alt-Backspace. I'm back at the login window.
If I select console login I can log in, run startx and all is OK.
The only errors in the Xorg.0.log file are
Could not init font path element /usr/share/fonts/X11/TTF/, removing from list!
Could not init font path element /usr/share/fonts/X11/OTF, removing from list!
Could not init font path element /usr/share/fonts/X11/CID/, removing from list!
but I have the same errors on this PC, which does not have this problem.
Suggestions appreciated. Thanks