I forgot to say that the file is in the root folder. I tested the v4 module and also the v3.1 "curled version", and I tried with many different roots with no success. Now I have this settings in the currency_update.php (of course replacing "my_user_name" with the real one):

define('/home/my_user_name/public_html/','/home/my_user_name/public_html/');
require_once(PATH_TO_STORE_ROOT . '/home/my_user_name/public_html/includes/configure.php');
require_once(PATH_TO_STORE_ROOT . '/home/my_user_name/public_html/includes/database_tables.php');