I know more people with this issue: After we upgraded to 12.04, we can't log out with Nvidia drivers enabled, I got a blank screen, sometimes with fuzzy words (like X server crash), and after some seconds the PC reboots. If I uninstall Nvidia drivers, I can log out without problems. I can't find any fix, I have tried with current and current-updates. Anyone have solved this? It's a bug? Thanks
The Nvidia drivers seem to have problems reloading during X server restart.
I faced this very same issue myself, using a GT520 card. The answer to me at least was posted on this forum:
For now, you can go around the issue by setting the Nvidia driver to persistent mode:
Fixed it for me.