hello I had the same problem as you ..
I clarify that I am using the google translator because I speak Spanish, and I registered just to tell you how it solved:
delete the folder /var/lib/pacman/sync/
and all its contents

# rm -R /var/lib/pacman/sync/

and then

# pacman -Syuf

anything send me a mail...
I hope it is solved Greetings from Argentina!

Quelle