Add to cart causes error page

I have been using your plug in for 4 months and I absolutely love it. I changed my theme today and now there seems to be a glitch. Each time you click the add to cart button, the page goes to a “this page not found” page. However, the item does get added to the cart and you can keep shopping but you have to keep hitting the “store” tab at the top of the website because you end up on error page. Is there a way to correct this? I tried creating a whole new page, but that didn’t seem to help. Thanks.
The site is http://www.robinkbennett.com

http://wordpress.org/extend/plugins/wordpress-simple-paypal-shopping-cart/