Can't add product to cart, see screenshot/url, help needed!

Hi,

So I'm using GridMarket, and MarketPress, one major problem.

When I register as a new user, so default user role is subscriber, I try to add a product to the cart but the load symbol spins then item(s) added, but nothing appears in the cart and on the product it shows as item in cart, but it should be available to re-add.

I have no clue why this is, it works fine if I use the admin account.

I've attached a screenshot, please see the url as well http://www.no1retroshop.co.uk/ and try it out, as I have no clue. Help is much appreicated!

Thanks!