i only see code on your pages to redirect to the cart page if the cart is empty:
?>
<?php
if ($eCart_de->IsEmpty()) {
$eCart_de->redirStr = "cart_de.php";
$eCart_de->cartAction = "RedirectIfEmpty";
}
?>
<?php
// WA eCart Redirect
if ($eCart_de->redirStr != "") {
header("Location: ".$eCart_de->redirStr);
}
?>
like i sugested before, make not of the domain in the address bar when it redirects to the cart page to make sure that the issue is not being caused by domain switching.