In V154 and 155. I have been testing with cookies refused. I have discovered that the Session id zenid changes every time you click on a link. I have tested this in fire fox and crome and both have the same issue. This I think then causes a whoops time out message to be displayed when you try to add an item to the cart.

I store the session info in the database and the session record is being created and destroyed for each new click.

You can see this if you turn off cookies and then go to the site then hover over a link you will see that the session id has changed for the next click.

I had to click once to get a zenid but from then on all the links have a different zenid.

I assume for a session to work the new links should have the same zenid (session id).