This bug affected me on a clean install of Ubuntu 20.04.
On upgrading another machine from 18.04 to 20.04 I found that I had both xscreensaver and xfce4-screensaver installed. This caused me to have to do a double unlock each time I wanted to wake the PC. I therefore deleted xscreensaver and got the same problem as on the clean installation, only worse. On the clean installation I could type the unlock password even though unlock-widget was not visible; on the upgraded machine I could not unlock at all.
I therefore removed xfce4-screensaver and re-installed xscreensaver. This fixed the problem. I now have two usable systems.
Some more info:
This bug affected me on a clean install of Ubuntu 20.04.
On upgrading another machine from 18.04 to 20.04 I found that I had both xscreensaver and xfce4-screensaver installed. This caused me to have to do a double unlock each time I wanted to wake the PC. I therefore deleted xscreensaver and got the same problem as on the clean installation, only worse. On the clean installation I could type the unlock password even though unlock-widget was not visible; on the upgraded machine I could not unlock at all.
I therefore removed xfce4-screensaver and re-installed xscreensaver. This fixed the problem. I now have two usable systems.