You should be able to take a look at the local checkout server behavior on the page to check to see where it is trying to go to. If it does not have the checkout success page listed as the success go to then there will be some problems. You can also try browsing to the checkout success file directly from your confirm page to see if you get taken back to the cart. If there is some code on your checkout success page that sends you back to the cart page you would experience something like this.
You can also do a find on your confirm page in the source code to look for a reference to your cart page, this would help to indicate how it is getting to this page. Let us know what you discover.
