Hey,
Is anyone else having this issue with 2.1? when you click on add to cart the ajax starts to fireup but the page then reloads without adding the item to the cart, with the loading ajax at the top still revolving… i even loaded back to a default skin but still the same issue.
JS error it provided is:
Error: response is undefined
Source File: js/ajax.js
Line: 178
Anyone have any thoughts on this?
I’ve just test on our site, using 2.1 and all work OK, Just as it did before 
Seems when upgrading multiple versions the htaccess file was not replaced with the correct one and was a problem in there causing the issue.
All resolved now!
I had the same issue, after I added the “required product” addon to a certain product. When I removed that option from the actual product, i was then able to add it to the cart.