Hi,
For a strange reason, when you are updating the cart, it calls an url ( /shop/product/updatecart/tmpl-component?from=module&product_id=13&cart_type=cart&quantity=1&add=1&Itemid=490&return_url=L2luZGV4LnBocC9zaG9wL2NoZWNrb3V0 ) and this url gave a "HTTP 303" to redirect the call to the checkout.
It is the reason why you see the checkout page in a part of your website.
Logically, you should have a "HTTP 200" and see the content of the cart module.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.