in paypal.php where you have the paypal form action url, the url was not match with the one paypal provides its customer. I just updated the form action with the current paypal form url. I recommend this to everyone who will let paypal be one of the payment methods.
this script was causing the problem:
$checkout_form_action = 'https://secure.paypal.com/xclick/business=' . rawurlencode(PAYPAL_ID) . '&item_name=' . rawurlencode(STORE_NAME) . '&amount=' . number_format(($total_cost + $total_tax),2) . '&shipping=' . number_format($shipping_cost, 2) . '&return=' . urlencode(HTTP_SERVER . DIR_WS_CATALOG . FILENAME_CHECKOUT_PROCESS . '?paypal_return=' . $paypal_return);
I replaced it with
$checkout_form_action = 'https://paypal.com/xclick/business=' . rawurlencode(PAYPAL_ID) . '&item_name=' . rawurlencode(STORE_NAME) . '&amount=' . number_format(($total_cost + $total_tax),2) . '&shipping=' . number_format($shipping_cost, 2);
and It works perfect.