not loading checkout page for products with product download

If I set up a cart with only products that have the "Product Download" field filled in then the page after viewing my cart (/store/shopping-cart/checkout/) doesn't seem to load anything after the page title. If there is at least one product that doesn't have a download then everything seems to work fine.

I checked the source for the page and it looks like the form is created but nothing is in it.
I checked the php error log and didn't find anything.

Our store can be found at http://oncourss.com/store/products/ (there's no cart widget yet so you'll have to go to the cart page manually: http://oncourss.com/store/shopping-cart/)

Does anybody have some idea of what might have gone wrong?