It seems you configured your system to only use mirrors from Greece, and it happens that one is out-of-date.
You may want to include more mirrors from neighbor countries:
# Worldwide list:
sudo pacman-mirrors -f 5 -c all
# Europe (uses geolocalisation)
sudo pacman-mirrors -f 5 -continent
# List available countries and use some
pacman-mirrors -l
sudo pacman-mirrors -f 5 -c <country>,<country>...