A couple of ideas, yes, but nothing conclusive. 
First things first, don’t use -Syyu. Use -Syu.
Secondly, if you are using btrfs, edit your /etc/mkinitcpio.conf, comment out all the lines at the bottom referring to compression, and then add the following line underneath… 
COMPRESSION=cat
Thirdly, refresh your mirrors before updating…
sudo pacman-mirrors -f && sudo pacman -Syu
Please report back on failure or success. 