The biggest blunders documented here:

lib32-audit:

sudo pacman -Rdd lib32-audit
then update your stuff. Background - that lib went into AUR, but had a back reference. (Manjaro only)

The infamous invalid pgp message
(mostly Manjaro, did not observe that at ARch or EndevourOS)

Manjaro - First try

sudo pacman -Syy
sudo pacman -S manjaro-keyring archlinux-keyring

IF THAT fails DO:

sudo rm -rf /etc/pacman.d/gnupg /var/lib/pacman/sync
sudo pacman-key --init 
sudo pacman-key --populate archlinux manjaro 
sudo pacman -Syyu

On real Arch Linux do:
pacman

 -Sy archlinux-keyring
Try update

Else:

sudo rm -rf /etc/pacman.d/gnupg /var/lib/pacman/sync
sudo pacman -S archlinux-keyring && sudo pacman -Syuu