Users get a "Shopping Cart is Empty" message

When users try to purchase items from my shop (all downloads), they get the message “your shopping cart is empty,” and they are unable to download. I have tested the cart in Firefox, Chrome, and Internet Explorer. It works fine with Chrome, but the empty cart message appears in Explorer and Chrome. I recently updated the eshop plugin. (Maybe last week or so. I’m not sure. I don’t know when the problem started. I received messages about it from two different people today.)

Thanks in advance for your help.

http://wordpress.org/extend/plugins/eshop/