keepassxc is a Qt based program. Try this fix mentioned in the stable update announcement:
Cannot mix incompatible Qt library (5.15.2) with this library (5.15.3)
- Check the mirror you use is up to date:
sudo pacman-mirrors -f 5 && sudo pacman -Syyu
- If you use qt5-styleplugins (AUR), rebuild it:
pamac build qt5-styleplugins