in the webassisxt/plugins/shopping_cart/pp_expessform.php file, change:
if (isset($_POST['PayPal_ExpressCheckout_button'])) {
to:
if (isset($_GET['ExpressCheckout']) || ($_SERVER["REQUEST_METHOD"] == "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST)) {