Hello !
This seems like something about your mirrors isn’t right, quite possibly related to the multilib repository.
I would suggest you to update your mirrorlist with the pacman-mirrors utility and additionally to force-update your repository cache with the command
sudo pacman -Syy
Aferwards, check if the system upgrade works normally.
If it does not, feel free to reply.
When you run pacman-mirrors using --country or --geoip a /var/lib/pacman-mirrors/custom-mirrors.json file is created. You only need to do this once or very infrequently, depending on the quality and existence of the mirrors.
When you use --fasttrack those custom mirrors are checked and /etc/pacman.d/mirrorlist is updated, which is the final list used by pacman.
When pacman runs, a fresh copy of the sync database in /var/lib/pacman/sync/ is updated. The double y updates all package databases. I believe that is saying update community.db, core.db, extra.db, and multilib.db. These commands aren’t executed that often, and it ensures databases are in sync. I’ve had fewer problems when updating since following this procedure.