In Xubuntu 18.x and Ubuntu XFCE 19.04, 19.10 (XFCE4, lightdm) I experience the un-lock screen does not show up. The screen remains locked!
Actually typing the password into the invisible lock sceen OPENS the lock!!!
That means the password request is working but does not show up. This is the reason why there are no error messages.
"light-locker-settings" explicitly tells me XFCE is controlling the "energy" settings. This works.
And there is no cover window over the GUI when it is "locked". The window in front of the main GUI does not show up (information not covered) and password entry does not show up.
QUESTION: How can I make the password request visible again??