GDM locks up: Unable to find session for cookie

[apotrebka]

Howdy everybody,

I'm seeing some strange behaviour on some Fedora 11 workstations for a client. The users log out of their workstations every night, in the morning, they come in, and the GDM desktop appears, but not the actual log in prompt.
When I SSH into the workstations, I see the following line in /var/log/messages

gnome-session[5636]: WARNING: Unable to determine session: Unable to find session for cookie

All the computers are updated (as much as possible, due to today's updates fiasco)
We're using the new version of the NVidia driver. (The same thing happened with the nouveau driver as well)
We use LDAP to authenticate our users, and mount their home directories via NFS.

If anyone could provide any insight on the issue, it would be much appreciated

Thanks alot!

apotrebka