Hi,
I have encountered some errors in your demo sites:
Occured on your www7 demo site when clicking on the "ADD OPTIONS" button of the testVariant product. Encountered today (wednesday 14sept) at 17h (for eventual event log))
Note that I encountered a Input string was not in a correct format too, but I am not able to reproduce it.
Occured on your www12 demo site when clicking the View Cart button after having done some other actions (add to cart, clear cart, proceed to checkout step the return...). Error encountered on Friday 9 sept.
I know these error are not detailed, as I have not found how to reproduce them with well-defined scenarii.
Moreover, note that the purchase simulation cannot be completed as it ask for a valid credit card number.