Since upgrading to Cartthrob 2.6 (on EE 2.7.0) I can only view the items in my cart when I'm logged into the control panel. When anyone else tries to add to cart they just see the page refresh and an empty cart. We're not using or creating members for the cart, everyone uses the cart as a guest.
I'm using the standard add to cart form with the return set to the same page. There is only one product but with a range of options.
{exp:cartthrob:add_to_cart_form return="samepage" entry_id="2720"}
<label>Card Value</label>
{exp:cartthrob:item_options entry_id="2720"}
{select}
<option {selected} value="{option_value}">${option_value}</option>
{/select}
{/exp:cartthrob:item_options}
...
{/exp:cartthrob:add_to_cart_form}
If I add to cart when logged out (and see an empty cart) - I can then log in, refresh the page and can see that the item is in the cart. If I then log out and refresh the page again, the cart shows as empty.
I've tried changing the redirection method in EE, setting the cookie domain... I haven't found anything that alters this behaviour so far.