What I don't understand: Why do you want the same user to login automatically after you just logged-out? This is probably the reason why it doesn't work the way you want it: there is simply no good reason to do this
Have a look in KDE Control Center -> System Administration -> Login Manager -> Convenience. There is an option for automatic login after the X-server has crashed. Maybe this will give you the option to login/out with Ctrl+Alt+Backspace.