The problem must be at your end, because I updated my system a few hours ago, and everything went fine. However, I also updated my mirror list before I ran the update — which one should always do when there is a bundled update.
The problem is most likely caused by the fact that one of the mirrors in your existing mirror list hadn’t (fully) sync’ed yet. If you still have one or several older pacman-mirrors versions in your /var/cache/pacman/pkg/, then I would recommend restoring that package from there… 
sudo pacman -U /var/cache/pacman/pkg/pacman-mirrors-{old-version-here}
… and then refresh your mirrors with that package…
sudo pacman-mirrors -f
… and run the update process again…
sudo pacman -Fy && sudo pacman -Syu
Another possibility, if you no longer have an older pacman-mirrors package in your cache, is to use manjaro-downgrade — if you don’t have it on your system, it’s in the repository — and to then repeat the steps of refreshing your mirror list and updating your system.
Lastly, see if this helps…