Shopping cart not getting updated even though product seems to have been added

I had successfully developed this site for a client and it was running perfectly on my system on localhost. I then uploaded it to my online server, where it seemed to be working perfectly and it still is: http://www.pikatzu.com/clients/mocean. The problem started after I uploaded it to the clients server: http://www.mocean.co.uk.

The problem is that whenever I add an item to cart. It shows the “Updating Cart” message, which should mean that everything should have gone through without problems. However, the shopping cart widget at the top of the site is not updated, and the checkout page is still empty.

I am wondering why this is happening. It is pretty strange as it is working perfectly on my test server. I had exported the sql and template files perfectly from one server onto another. I have also refreshed the cache, flushed my theme files, deactivated all my plugins, changed my theme, but none of this seems to have worked at all. Any help would be greatly appreciated.

p.s, I also did a javascript function that refreshes the page anytime the “Add to Cart” button is pressed. I am sure that its not from this as I have removed it and tested it without this function. Furthermore, it is working perfectly on the test server and not on the clients site.

http://wordpress.org/extend/plugins/wp-e-commerce/