I upgraded to 12.10 some days ago and now I'm unable to change between the environments. I currently have GNOME 3 (with extensions), KDE and, of course, Unity installed. The computer is a Lenovo G550. In the login screen I can get into the menu where the DEs are listed, but I can't select one. I can't use a test account of mine, which used GNOME 3, because after I logged in X crashes/restarts. Unity works, but I can't switch from the broken GNOME 3 to Unity. What did I broke during the upgrade and how can I fix it?