New Customer 'CONTINUE' Button at Checkout is Not Working

Hi there, I'm running 1.5.3.1 and a customer just brought the following error to my attention and I have never seen it before. If an unregistered user goes to hit the 'CONTINUE' button to create a new account on the checkout screen, I get the following pop-up... seems like a file is missing but I have no idea what it's actually looking for.

Any insight would be greatly appreciated. Thanks in advance.

Forgive my noobness, but where do you actually see that the request is made to index.php?route=checkout/undefined ? I'm not really understanding how these buttons work because it's not like they are a standard link utilizing an anchor tag and the 'href' attribute.

Anyone know off the top of their head where this button should actually be taking the user? Should it just be going to index.php?route=account/register or is it something else?

Thanks again...