I am having a similar issue. Everything was working great after testing last night and then this morning I could not go from Step 1 to step 3 in the cart. I have my shipping info set and all of that is OK but the button is looping back to checkout_shipping instead of checkout_payment! Rest is working fine when I make the url go to checkout_payment.
Help, please - I have not been able to figure this out in any of the code although I can identify where the files are located.
My cart is therefore broken for the client and she is going to have my head in another minute.
www.soulsandsenators.com/zencart