Hi,
Thanks, I finally got it.
Please edit the plugin "plugins/hikaserial/attachserial/attachserial.php" and replace the code
$orderClass = hikaserial::get('shop.class.order');
$order = $orderClass->loadFullOrder($file->order_id);
With
$orderClass = hikaserial::get('shop.class.order');
$order = $orderClass->loadFullOrder($file->order_id, true, false);
The function "loadFullOrder" of HikaShop generally check that the current logged user have the right to open the order (in the front-end context).
Because you're using a guest account, the user is not logged so HikaShop can't see that the current user is the same than for the order.
That's why we have to deactivate the check of the user with the third parameter ; and because the guest download was not available before, I didn't remove the user check before.
Regards,