Stopped working – getting 404 error when Submit Payment button pushed

Here’s a link to test: https://www.toolbox4healthcare.com/mindeasebilling/

Ok – so more weirdness – The Cart page, that the buy now button is supposed to redirect the user to – IS THERE- but for some reason, when you get to it from the add to cart button, its a 404 error, yet if you cut and paste that same url into a new window, the page shows up. WHY OH WHY???