in wiegant_ideal.php

find:

$website = HTTP_SERVER; // The url pointing to your shop
$success = HTTP_SERVER ."/checkout_process.php";
$failure = HTTP_SERVER."/checkout_payment.php";

change to:

$website = HTTP_SERVER; // The url pointing to your shop
$success = "/checkout_process.php";
$failure = "/checkout_payment.php";


Because later in the script, HTTP_SERVER; is called again like HTTP_SERVER. $success, making it double and ****ing up the return URL.

More...