This fixed the situation on my machine (no explicit key refresh! as suggested above by @mithrial , and I had removed problematic sigs/archives from /var/cache/pacman/pkg in previous attempts to rectify the situation)
sudo rm -r /etc/pacman.d/gnupg
sudo pacman-key --init
sudo pacman-key --populate manjaro
sudo pacman-key --populate archlinux
sudo pacman -Syvv manjaro-keyring # just to test for key-problems, as this would fail
After that, I used pamac (gui) to update everything - the machine is up to date now. Helmuts key is valid and not expired on my machine (expires never!), that’s why I think not to refresh the keys manually is important for now – once the situation is cleared up, normal procedures apply (see below)