dasonix,

I've just checked and if you set the callback URL to:

Code:
https://<wpdisplay item="MC_callback">
you must also make a change in

includes/modules/payment.worldpay.php at line 129

from:

Code:
$worldpay_callback = explode('http://', $callback_url);
to:

Code:
$worldpay_callback = explode('https://', $callback_url);
Hope this helps,

Alan