Pacman Recovery
Due to issues with pacman updates and the system freezing, logging out etc. it has become necessary to recover from this more than once.
The critical stages are
sudo rm /var/lib/pacman/db.lck
Then find what is not complete
LC_ALL=C pacman -Qkk 2>&1 | grep -v ', 0 altered files' | grep mtree
Then reinstall packages returned from the above with
sudo pacman -Syu <package> --overwrite '*'
The last command may prompt removal of an incorrect package in the cache, so delete it and run it again to download a new complete package.
After all the errors are gone you can move on and hope this is fixed soon.