When attempting to register an account today, I would be redirected back to the checkout cart page every time. The page was making an ajax request to checkout/onepage/savemethod and it would return a 404. This only occurred on our staging server and not locally, so I figured it must have been a configuration difference. Locally where it worked, checkout/onepage/savemethod correctly returned a blank page and not a 404. Long story short, the solution was to log into the admin and to go configuration->sales->checkout and enable “allow guest checkout” . A more elegant way of handling this would be good, like a user message on the frontend stating what the problem is instead of just returning a 404 and a redirect.