I have an issue where if a customers adds a product to their cart and then goes to view the contents in their shopping cart and tries to change the quantity of products in their cart they are redirected to a page that states product not found. This happens for every product on the website. Every time a customer hits the update cart button, they are redirected to a page that says Sorry, the product was not found.

This is a link to one of the many products on our website. Again the problem occurs with all of our products.

http://westechequipment.com/product/...max-gpm-1-3-hp

Any ideas as to why this might be happening?

Four weeks ago we upgraded to the latest version of Zen Cart v1.5.5a from 1.5.4 following the official upgrade procedure.

Our server is using PHP version 5.5.36 and MySQL version 5.6.30

Thanks!