1) I don't see the code for turning on error reporting.
Did you try that? Was there an error?
2) Try changing line 1090 - 1092:
else if (($_SERVER["REQUEST_METHOD"] == "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST)) {
$WA_Store_Cart->redirStr = "checkout_success.php";
}
else if (isset($_POST["Submit_order"])) {
$WA_Store_Cart->redirStr = "checkout_success.php";
}