Another upgrade/uninstallation problem: when my screensaver kicked in, the password prompt would no longer appear when I pressed a key on the keyboard. I was locked out.
In /var/log/syslog: cs-backup-locke[2921]: unable to monitor screensaver window: Failed to execute child process “xprop” (No such file or directory)
$ dpkg -S xprop nothing
$ apt-cache search xprop says something about x11-utils
$ sudo apt install x11-utils
Then try locking the screen… and it works!
By the way, to bypass your broken lockscreen: get into another terminal
(control-alt-F1 or ssh or whatever) and cinnamon-screensaver-command -e. Then
you can return to the graphical terminal, wiggle the mouse, and get your desktop
back.