So, after seeing a fellow forum member’s post about an issue with Plasma, I thought I would install Plasma alongside my Gnome desktop, to see if I could help them solve it. I’ve done this successfully before and then removed it again, without issue. But this time, during the first login to the Plasma session the machine locked up and I ended up having to do a physical hard power-off 
Upon booting up again, the filesystem check had to recover the journal and fixed a few issues. Then the fun began… Plasma/KDE packages installed but not installed according to pacman, empty files from those packages preventing the packages being installed again, ldconfig warning of empty files…and so on!
A few hours later I had all that resolved, but one very confusing issue remains. When the desktop locks (either Gnome or Plasma) I cannot unlock it again! In Plasma, it says “Unlocking failed” and on Gnome it just blinks and then asks for my password again. I can use the “loginctl unlock-sessions” command from a vtty terminal to get the desktop session unlocked again (with the same password as what fails in Gnome or Plasma lock screen), but just can’t unlock “graphically”.
Anyone have any ideas where to look?
Edit: I don’t think that is my only problem actually… kwin_x11 seems to want to start using ~100% CPU after a while. I’m sensing a full reinstall is in my future…