Unable to switch user after upgrade to 24.04

Question

I just upgraded from 23.10 to 24.04, after the upgrade dialog popped up. Since the upgrade, I'm unable to switch users.

At the login screen (lightdm), I can select any user, but once a session for the selected user has started, clicking on another user name from the list under "Lock/Switch account" has no effect. I also cannot log out and select another user. Selecting "Log Out" ends the session, but no login screen appears, I just see the Noble Numbat background screen and the mouse pointer. (Edit: actually, the login screen does appear after a while, see below.) The lock screen is working, though.

The final two lines of .xsession-errors are:

Failed to get properties: Unit name [email protected] is neither a valid invocation ID nor unit name.
Failed to get properties: Unit name [email protected] is neither a valid invocation ID nor unit name.

So possibly something with gnome-session-manager is broken. However, these two lines are already in .xsession-errors before I try to switch users.

Any ideas what to do?

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Edit: When selecting "Log Out", the login screen does actually reappear, but only after 90 seconds, as if some timeout was triggered. No further hints as to what is wrong in .xsession-errors, .xsession-errors.old, Xorg.0.log or Xorg.0.log.old.

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Edit 2: FWIW, I have found kind of a workaround for the problem:

• At the lightdm login screen, choose a different desktop (e. g. "Ubuntu on Xorg" instead of the default) and log in as some User A.
• Lock the screen. You should now be able to log in as some other User B.
• Go back to User A's session (Ctrl-Alt-F7) and log out. You can now log back in as User A with your preferred desktop.
• Now you have two sessions running which you can access with Ctrl-Alt-F7/8.

For me, the default desktop is Unity. I noticed that there is a dedicated flavor of Ubuntu just for Unity (https://ubuntuunity.org/). However, I installed from the default Ubuntu image, so maybe that was a mistake.

I added unity to the question tags as the underlying problem is likely somehow connected to it, given that "Ubuntu on Xorg" works.