change line 137 - 142 of the product_detail.php page from:
<?php
// WA eCart Redirect
if ($WA_Store_Cart->redirStr != "") {
header("Location: ".$WA_Store_Cart->redirStr);
}
?>
to:
<?php
// WA eCart Redirect
if ($WA_Store_Cart->redirStr != "") {
session_commit();
header("Location: ".$WA_Store_Cart->redirStr);
}
?>