Not sure why, but I tested on default template that no product goes into cart, just gives the error saying “Product could not be added to cart.”
Also when logged in and going to cart I get “Redirect Loop” and 404’s!
When not logged in cart is shown as empty, but as said nothing can be added.
So this upgrade makes the store unusable, since noone can buy anything! BAD!
All products tested are configurable products.