sudo pacman -Rns polkit
sudo pacman -S polkit
You might also try resetting permissions as indicated in post#4:
Fix
polkit-1directory and file permissions:sudo chown -R root:root /usr/lib/polkit-1/ sudo chmod 755 /usr/lib/polkit-1/ sudo chmod 755 /usr/lib/polkit-1/polkitd /usr/lib/polkit-1/polkit-agent-helper-1 sudo chmod 644 /usr/lib/polkit-1/rules.d/*
Reboot.