Plasma desktop can't unlock session?
If you tried this topic and didn’t help, you’re probably using gentoo. I came across this bug with ~5.3.1. Stracing gave me following sequence:
What the hell? Looking for kde pam module and skips right to “other”, which denies auth without a question. If you have the same problem, try symlinking: ln -s /etc/pam.d/system-auth /etc/pam.d/kde