Order complete returns a 404

[GOT-Hosting]

After an order has been made, and you are on the last screen, if a customer clicks the continue button, they get a 404 error, even though the page in the address bar is a valid page:

 

checkout_success.php?action=update

 

Now, with that said, it is probably important to point out that this cart is using a shared SSL certificate, so the SSL is on a different domains and uses the ~username format.

 

Any help greatly appreciated!

[GOT-Hosting]

Anyone? :)