Unity 3D refuses to start here, always falling back to Unity 2D. I installed both Gnome and XFCE 4 and neither starts, but instead leave me with the mouse cursor and the wallpaper. I want to know what is happening. Where are the session manager logs? Where can I see what processes failed to start? Where can I read what they printed to stdout and stderr before they crashed or failed to start properly?
The file you should take a look at is
If there are issues with the greeter/LightDM, then you can go through the logs in