I just had this very same error. The problem seems to be that there are new keys in the archlinux-keyring package, and new packages (confuse) signed with those keys. Since both packages are updated in the same transaction, well the new keys cannot be used until the update is finished, but the transaction will not start until the packages are checked…
The solution would be to update the archlinux-keyring by itself:
pacman -S archlinux-keyring
And then do the rest:
pacman -Su
If that fails, you could try running through the keys manually, with:
pacman-key --populate
but usually, it is not necessary.