I just received a similar error message, for me it occurred because I tried to run an update earlier in a network that required me to login.
So somehow the HTML login page landed in my sync files for core, community, multilib here:
Simply check if a similar thing happened to you:
$ cat /lib/pacman/sync/core.db
For me, simply removing these files was sufficient to fix the
If you're unsure copy them to a safe location prior to that.
$ sudo cp /lib/pacman/sync/core.db ~
$ sudo cp /lib/pacman/sync/multilib.db ~
I guess there is also a pacman command for clearing the cache wich should work aswell.