Comment 8 for bug 1597719

I confirm Linux Mint 18.2 is also impacted, started off with Mint 18.2 Mate then installed KDE+plasma-desktop on top of it and I get the same irritating behavior when lock screen is performed - black screen with message "The screen locker is broken and unlocking is not possible anymore.
In order to unlock switch to a virtual terminal (e.g. Ctrl+Alt+F2),
log in and execute the command:

loginctl unlock-sessions

Afterwards switch back to the running session (Ctrl+Alt+F9)."