Proceed to Checkout button empties cart

I use woocommerce. When clicking on ‘Proceed to Cart’ the cart is emptied. It is behaving like an Empty Cart button. Links to the checkout page work fine.
The site is https://quotethewalls.com
What would cause this? It uses site wide SSL except for a custom order page. When I remove all redirects from htaccess it doesn’t make any difference.
I’ve tried to remove the button entirely with a remove_action in functions.php but that didn’t work.
Any ideas?