New Post: Ciritical error when adding product to checkout or going to the checkout page

I'm examining Poly and Kiala dresses as broken and working examples. The XML is essentially identical.

Try removing all categories from both, then create a new test category and add them to that. See if they behave the same.

Try copying the faulty one through the store's own copy product button and then progressively remove each option until it either works or you're back to no options at all. Then take the images off, and the categories etc. Basically just trim it back one thing at a time until it works.. it must surely start working at some point.
Rob