Comment 6 for bug 2020245

It is a problem with Lubuntu and how the screensaver daemon is started. I don't see here in the report which version is affected, but since a long time the package comes with a desktop file /usr/share/xscreensaver/xscreensaver.desktop referencing its own wrapper script /usr/share/xscreensaver/xscreensaver-wrapper.sh to start up. This wrapper script tries to identify what kind of session it is started in, and to set the "user-switcher" accordingly. It should set it to "dm-tool switch-to-greeter" if lightdm was used to launch the session.

It is quite difficult for the xscreensaver daemon (or the wrapper) to know what is the correct user-switcher command to call. It makes a lot more sense that the desktop session tells xscreensaver which one to use when it launches the daemon...