If I recall correctly (it’s a few months ago..) it was an url rewrite issue with the location of the cart and billing page. I put the pages inside the ‘shop’ page:
Example: https://www.url.com/shop/billing/order-received was not working and when I changed it to https://www.url.com/billing/order-received, it worked fine.
I am not entirely sure this was the exact case, but worth trying!
