I was able to reproduce this behavior with running manually /usr/lib/x86_64-linux-gnu/libexec/kscreenlocker_greet
Oddly enough the first time I ran it I did got the password prompt and was able to unlock it and return to my desktop.
Running it again reproduced the reported issue, i.e. no password widget and thus no way to unlock it.
Killing kscreenlocker_greet from an other VT, didn't fix the issue, rather background became black, and only the mouse pointer was moving around.
I was able to reproduce this behavior with running manually /usr/lib/ x86_64- linux-gnu/ libexec/ kscreenlocker_ greet
Oddly enough the first time I ran it I did got the password prompt and was able to unlock it and return to my desktop.
Running it again reproduced the reported issue, i.e. no password widget and thus no way to unlock it.
Killing kscreenlocker_greet from an other VT, didn't fix the issue, rather background became black, and only the mouse pointer was moving around.