in the webassist/plugins/shopping_cart/pp_expressform.php file, change the following code:
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))
in the webassist/plugins/shopping_cart/cart_editable.php file, i think you can comment out line 135 to prevent the error in the logs:
//mysql_free_result($Taxes);