Alright, I finally spotted the problem and fixed it.
You have to edit two files, the first one is currency.php in administrator\components\com_hikashop\classes replace this line:
if(empty($products[$key]) || !in_array($product->product_id,$products[$key])){
with this one:
if(empty($products[$key]) || !in_array($product->product_id,$products[$key]) && !in_array($product->product_parent_id,$products[$key])){
then edit the product.php file in the same folder and replace this line:
if(isset($element->$field) && is_array($element->$field) && count($element->$field)){
with this one:
if(isset($element->$field) && (is_array($element->$field) && count($element->$field) || is_object($element->$field))){