After upgrading KUbuntu 12.04 through KDE Backport ppa and restart login screen appeared normally. Unfortunately only black blank screen appeared after login procedure. Previous version was absolutely healty 4.10 KDE. Reproducible: Didn't try Steps to Reproduce: 1. Add KDE Backport PPA to software sources 2. Run and accept update 3. Restart computer Actual Results: Login screen appears, but black blank screen instead of plasma desktop is shown after successfull login. Expected Results: Plazma desktop should have appeared. After several restarts with same result I entered tty1 terminal (CTRL+Alt+F1), logged in and installed plazma-desktop sudo apt-get install plazma-desktop After restart everything was back to normal again. I must note that during the initial upgrade no errors occured. Somehow plazma-desktop was not installed.
Downsteam packaging/upgrading issue. Please report the problem to (k)ubuntu developers