I have a same kind problem. I'm testestin system now, and when checkout came to page wher customer can check his/her cart a last time, there isn't a "next-button" If I remove a cart from checkout, all will work perfectly (But can i really remove cart from checkout-system... no.)
However, I created a brand new test customer. In that account, checking out wil work normally.
Problem with coocies, locally or server side?
Edit: Second try to "buy" something, (after a cancelled withdrawal) again there is no "next"-button in "Cart"-page.
SOLVED, I think. Seems there is some errors in product weight or/and volumes and that make mistake to cart-secreen. (???)