Google Checkout Notification Failing Problem

I'm currently using this module:
http://www.opencart.com/index.php?route ... sion_id=18

I've followed all the instructions and Google is clearly processing my orders because I get order notifications in my mailbox, but the order does not follow on through to my shopping cart. Instead, I get this error email message:

We've made several unsuccessful attempts to send order notifications to your notification callback URL, https://www.(mystore).com/index.php?route=checkout/gcheckout/callback As a result, you aren't receiving order status, risk, or other notifications.

Does anyone know how to solve this problem?

Oh, and I'm using Opencart Version 1.4.9.3. The extension is the latest version.