Just hit the same issue with a fresh server install (from test ISO).
The default mirror has some corrupted database files right now. I will be moving packages soon which might fix the issue, but I will need to contact the mirror owner to see if he can correct.
In the meantime, best option is to open the mirrorlist:
and comment out the redcore (first) mirror. Make sure your preferred (closest most often) is the first listed in mirrorlist.
After editing the mirrorlist, you will have to make sure the corrupted database files are no longer in
sudo rm /var/lib/pacman/sync/core.db
sudo rm /var/lib/pacman/sync/main.db
sudo rm /var/lib/pacman/sync/apps.db
To see the status of all mirrors: