Hi,
When I click on the add to wishlist button there, the AJAX request for that action is sent to the server. However, the server answer with that response:
monosnap.com/file/u0Ud0k0IcVxqHWLe7bIJr8jw8CYNuX
As you can see, there is apparently an issue in one of the files of your Joomla template because apparently it doesn't handle properly the "tmpl=ajax" of the URL, which is a standard parameter of Joomla (albeit not used much usually).
As a quick fix, you can rename that /templates/lt_restaurant/error.php file to deactivate it and it might help.
Best regards,
Nicolas