Ajax response: http error: 200

Hi,

1. Customer open 2 browser's cards with product view.
2. In first one, he loggin.
3. In second one he click 'Add to cart' (without reload page).
4. Product does't add to cart and customer see a popup message:
An error occurred in AJAX HTTP.
HTTP Error 200
Diagnostic information.
Path: / system / ajax
StatusText: OK
responseText:

How to repair this. Customer shouldn't see this error message.

Regards