I've been using KDE for very long time (except when it lost all of its stability in the beginning of KDE4). Lately, since everything was going smooth with my NixOS setup, I've decided to remember my good old window manager days and try out some tiling WM options. After all I am already using an extension for KWin to have tiling feature. First tried dwm since my extension tries to replicate its layouts anyway, but patching the C code for every change wasn't for me.
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