Qt troubleshooting

Cannot mix incompatible Qt library (X.X.X) with this library (Y.Y.Y)

This could be qt5-styleplugins, try rebuilding/reinstalling.

Reinstall older qt lib

Maybe not all of your applications use the new one yet.
here's a one liner to target them.

# identify/install the latest version under 5.15.10 for every qt lib
for f in `ls qt5-*-5.15.10*.tar.zst | sed -E 's/(qt5-[^-]+).*/\1/' | sort | uniq`; do
    sudo pacman -U $(ls $f-5.15.10*.tar.zst | sort | tail -n 1)
done