bug with checkout after entering a coupon code

I'm attempting to buy AFX2, but when I enter the coupon code and click 'go', the background turns white and the only thing displayed (in plain text) is what I have in my shopping cart. All the buttons go away. I have tried going back to the main page and re-entering the checkout, but it stays that way.

I have also tried clearing my cache and have tried it on firefox, IE and chrome to make sure it wasn't a browser issue, but got the same result.