Looking at that log from the Arch forums, seems like a QML compiler bug? I’m not sure if clearing off the .cache folder would solve it, but if I had the issue, it’s the first thing I’d try. Stale caches after a plasma bump are notorious for generating weirdness. Hopefully fixed soon.
Clearing the fontconfig cache seems to have worked for me using the latest fontconfig; if someone else can confirm it:
rm -rf ~/.cache/fontconfig/
systemctl --user reset-failed plasma-plasmashell.service
systemctl --user restart plasma-plasmashell.service
Edit: tried it on my second affected system without any downgrades, and it seems to work.