My first guess is that the mirrors used by this installation haven’t been updated or refreshed for a very long time, either.
You might try (preferably from a TTY entered outside of the GUI);
pamac update --no-aur --force-refresh
Wait for that to successfully complete, and then reboot. All being well, that might allow future updates to complete.
A brief mention that mirrors should ideally be refreshed as a part of regular maintenance; an example using pacman-mirrors is:
sudo pacman-mirrors --continent
which limits the selection to more local regions.
- pacman-mirrors (Manjaro Wiki)
From a previous Update announcement;