It doesn't look like the site is erroring on anything, as it's showing the complete page with the footer (if it errors, you normally see an abrupt end to the HTML output).

So my best guess here is that there is something wrong with the view.php for your theme. The cart/checkout/billing/payment pages all use the file view.php from your theme, as they are 'single pages'. In the view.php there should be a line:

(sometimes you'll see print instead of the word echo)

If that isn't there, you'll be missing the forms and what-not of the checkout.