Hm, do you also have another than KDE desktop ?
My solution would be a bit drastic, though it should work, it would be to totally uninstall KDE and then reinstall it from scratch :
1/ Using MCC's software manager, have another desktop installed, like gnome, blackbox, whatever. So that you may totally screw up KDE and still be able to log onto a graphical system
2/ Inside the MCC, check if you're using kdm, and if yes, chose another manager, like gdm or mdk-dm.
3/ Using MCC's software manager, totally uninstall KDE, and I mean it, totally, all programs with KDE in their name should be removed - start with kde-base so that all kde dependencies will be auto-selected.
4/ if you don't have important personalized settings, even remove ~/.kde folder
5/ then, running gnome or another desktop, run the kiosk installation of your latest KDE, this time it should work...
... well, I hope