Verify that your system’s time and date are corrrect
You can try to reset gpg keys database:
sudo mv /etc/pacman.d/gnupg /etc/pacman.d/gnupg.old
sudo pacman-key --init
sudo pacman-key --populate archlinux manjaro
sudo pacman-key --keyserver hkps://sks.pod02.fleetstreetops.com --refresh-keys
There is currently a problem with the default keyserver for gpg. I recommend to do this:
sudo sh -c 'echo keyserver hkps://sks.pod02.fleetstreetops.com >> /etc/pacman.d/gnupg/gpg.conf'
echo keyserver hkps://sks.pod02.fleetstreetops.com >> ~/.gnupg/gpg.conf
After that try again to update the system:
sudo pacman -Syyu
Edited: Fix typo