Unity does not switch between users after upgrade to Ubuntu 18

Question

I have upgraded Ubuntu 16 to Ubuntu 18. I do not intend to make a further upgrade to Ubuntu 20 for the moment. All looks to have gone well except the switching of users from within the desktop environment. My desktop environment was and is Unity.

There are two users in this desktop. The expected behaviour is that I can switch to any other profile without logging out of the current one, like in a multi-user system.

Initial disruption

The situation straight after the upgrade is

• I can log in into both desktop profiles separately (pass).
• Once I am in either profile and click on the menu to access the other, nothing happens (fail).

So I stay where I am. I need to log out completely in order to access the other profile.

Partial fix, new disruption

I followed the suggestion to reinstall the desktop environment with

sudo apt remove lightdm
sudo apt install lightdm
sudo reboot

This fixes the issue only partially.

• I can log in into either profile separately, as before (pass, old).
• If I click on the menu inside a profile to access any other profile, something happens: I am presented with the login screen with the password input (pass, new).
• The resolution of the login screen where to input the password is coarse (fail, new).
• After I type in the password, it hangs for several seconds before it loops back into the log-in screen. I cannot access the other user profile (fail, new).

Safety checks

• This always happens with the second profile, regardless of which the first profile is.
• The problem is limited to the desktop environment. I can log in from the terminal into both profiles.
• There is no issue with encryption: I can see the contents of the home directory from the terminal.
• If I log out of the first profile and start afresh, the log-in screen with the password input is rendered neatly and I can log into the other profile.

Question

Therefore, the problem seems to be purely one of managing the switch from any current profile into any other profile.

Basically, I always have to log out from one profile to log into the other. So no multi-user behaviour.

Any suggestions to fix this?

Other posts

I have looked into these other posts, which refer to older releases of Ubuntu

• Unable to switch users in 13.10
• Ubuntu cannot switch user
• Switch user after upgrade to 14.04 not working