Ubuntu :: GDM Login/switch User Screen Blank - Autologin Works

I've run into a strange problem with GDM that I haven't managed to find a solution for yet, either by trying myself or googling, and I have run out of good ideas. I'll just infodump here about the problem and what I've tried etc;

I installed Ubuntu on a new PC a few weeks ago, setting it up with autologin for my mom and a separate user for myself, using the on/off-icon menu in the top-right corner to switch to my own user and back as needed, and logging the user out when done. This worked quite well.

However, a few days ago, this stopped working; logging out or trying to switch now leaves me with a blank black screen, without even a mouse pointer (but not off, the backlight is on).

EDIT: To clarify, this is an issue that only affects the GDM login screen, but that affects it whenever it is used, wether it is on boot (when not set to autologin), after logging out, or when trying to switch user.

At this point, I can usually use Ctrl-Alt-F1 to get a textmode login, and Ctrl-Alt-F7 to get back to the auto-logged in session (assuming I tried to switch, not logout).

I can't think of anything specific I did or installed around then that should be related in any way...

I tried disabling autologin and restarting gdm, which left me with the same black screen (which is still there after rebooting), instead of the expected login window. I managed to re-enable autologin by manually editing the /etc/gdm/custom.conf file, so that it would at least work for mom.

I've also tried to change which user is auto-logged in, thinking it might be a problem with my user account, but both users get an automatic session just fine when I restart gdm.

I thought it might be a problem with the video driver, but that's not the case - if I run zenity --info as root with DISPLAY set correctly, the dialog box appears on the screen just fine. It has no borders or titlebar (there's no windowmanager), and is apparently without keyboard focus, so since there's no visible mouse pointer I can't click the OK button... But since it appears, X is apparently up and running just fine, just has nothing to display other than a black background.

I tried purging and reinstalling gdm and gnome-session(-bin|-common), but that didn't help any.

Running ck-list-sessions after trying to switch indicates that there's a new session there, with session-type = 'LoginWindow', so it appears to think everything's fine.

Enabling debug output in the /etc/gdm/custom.conf file did get me some more debug output in the gdm logs, but it didn't really tell me anything, there weren't any obvious problems that I could see.

After some looking around, I've guessed that it's supposed to be running gdm-simple-greeter, which I assume would display a login box; trying to run it manually doesn't work though (it's missing some environment variables, and trying to add them based on the abovementioned debug output doesn't really help).