Checkout button redirects to Product Listing

-- url of the page with the problem -- : seven10mxgraphics.com/hikashop-menu-for-...ion-dark-blue-orange
-- HikaShop version -- : 2.6.0
-- Joomla version -- : 2.5
-- PHP version -- : 5.3.2
-- Browser(s) name and version -- : Firefox 42.0
-- Error-message(debug-mod must be tuned on) -- : No error on page

When I add a product to the cart it should redirect to the checkout page but now it turns to the product listing page.
This have been happening to me few weeks ago. I updated to the last version of the HikaShop but nothing happens. Also I installed a clean joomla site and installed the HikaShop Component but I'm still getting the error.

Thanks Nicolas,

I have tried everything, I changed the way joomla handle the session and the problem persists. In fact I bought a new web hosting in godaddy with the lastest joomla version and the lastest Hikashop but when I try to check out I get the error "The cart is empty" and the page takes me to the Product listing.

Am I missing some configuration? The thing is that my site was working fine until 2 months ago.

I really appreciate you help. Thanks!