Hello,
Okay.
Regarding the URL, it looks like the fact that there is no "order_id" in the parameter, HikaShop is not able to determine what is the current order id to determine which payment plugin load to get his cancel url.
But HikaShop should have the "order_id" information in the current session ; so that is strange that it cannot retrieve it...
In the hikashoppayment mangopay plugin, you can replace the line
$redirect_url = HIKASHOP_LIVE . 'index.php?option=com_hikashop&ctrl=order&task=cancel_order';
By
$redirect_url = HIKASHOP_LIVE . 'index.php?option=com_hikashop&ctrl=order&task=cancel_order&order_id=' . (int)$order_id;
In order to force the "order_id" in the URL and then HikaShop should continue his normal process.
Regards,