[MarketPress] Cannot add products to cart.

I cannot add a product in the MarketPress cart. It looks like the add to cart button is sending the ajax request to an invalid URL in terms of protocol (http, should be https) and address (missing site URL). I see the following error in the console :

Mixed Content: The page at 'https://wp+++++.dk/store/products/josefin/' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://wp-admin/admin-ajax.php?action=mp_update_cart'. This request has been blocked; the content must be served over HTTPS.