Hello, Thank you for the forum and the shows. Here is my issue.
Sometimes, after “Switch user” or “Login as another user”, the system will not allow login with another user. The only way out is to login again as the previous user and Log out.
Ubuntu 18.04, updated; Gnome; X11
As an attempt to improve the behaviour, I uncommented WaylandEnable=false in /etc/gdm3/custom.conf. I also refrain to start a session with Wayland. It seems to make things better, but the problem still appears every now and then.
Note that at some point, I installed XFCE, with which the problem was probably worse. (I uninstalled XFCE since then.) The behaviour is now seemingly random and not very frequent. At some point, I also experimented with lightdm instead of gdm3 but it didn’t seem to help much.
Any input or pointer welcome. Regards.
PS: In practice, the “next” user facing that situation will often have no other option than rebooting the system 
May have to hit the Ubuntu forums for this issue.