Hi,
It's actually a bug of that popup when the canonical URL of the product is filled.
Change the code:
echo '<textarea style="width:100%" rows="5"><a class="hikashop_html_add_to_cart_link" href="'. hikashop_contentLink('product&task=updatecart&qty=1&cid='.$product_id, $product, false, false, false, true).'">'.JText::_('ADD_TO_CART').'</a></textarea>';
to:
$menusClass = hikashop_get('class.menus');
$id = $menusClass->loadAMenuItemId('','');
if(!empty($id))
$id = '&Itemid='.$id;
echo '<textarea style="width:100%" rows="5"><a class="hikashop_html_add_to_cart_link" href="'. hikashop_completeLink('product&task=updatecart&qty=1&cid='.$product_id.$id, false, false, false, true).'">'.JText::_('ADD_TO_CART').'</a></textarea>';
in the file administrator/components/com_hikashop/controllers/product.php and it will then provide a correct link in the HTML of the popup.
We'll add that patch for the next version of HikaShop which is to be released soon.