eCart - The Cart Is Empty

I have followed the tutorials of implementing eCart. I'm not using a payment Gateway, and purely sending the order by e-mail. Everything is working perfectly on my local testing server.

However, on the live server I am getting inconsistent results. On the product page, when I click on "add to cart" I am redirected to the cart page with an error - the cart is empty. When clicking on the back button to the products page and I use the search functionality, and then click on the "add to cart" from the results products, I do not get the cart is empty error. When I click to continue shopping and select another item to the basket, I only get the latest product listed. Not the previous item added to the cart.

I believe the above errors are somehow related to another error on the same page, being: search by category and keyword. I have posted the error on the DataAssist Forum.

I have included my working files and look forward to some feedback and solutions.

A link to this project is:
products_Results.php