My problem is that when I log out from a gnome shell session I get a blank screen instead of the display manager. If I close the lid to suspend and then open it again, I get the display manager and I can log back in.
This does not happen with Unity. When I log out from a Unity session I get the display manager without any problem.
I'm running Ubuntu 11.10 64bit, with gnome-shell version 126.96.36.199. I already checked
/etc/X11/default-display-manager, and it's set to
/usr/sbin/lightdm, as it should be, according to this post: http://askubuntu.com/a/84485/61336.
This is the exact same problem described here Ubuntu 11.10 with Gnome Shell does not logout properly, but as you can see, I provide some additional information. I tried to leave a comment in the original question to avoid a duplicate, but I don't have enough reputation. I also tried to ask in the meta section what to do in order to avoid a duplicate in this situation, but again, not enough reputation.