Order Status Not Changing After Payment & Order Received page 404

Hi,

I noticed today that orders were coming in and marked as Payment Pending but the payment was sent in Paypal.

I did a test order to see what was going on, and I got this:
http://i.imgur.com/ABUUpUT.png - a 404 error on the order received page

I'm really not sure how to diagnose what the issue might be. Could you provide me with some help to debug this issue please?

Thank you