Hi,
In the class order (administrator/components/com_hikashop/classes/order.php) there is two references to "$order->order_invoice_id".
Please update the file with these two new indicated lines:
$order->order_invoice_id = $this->database->loadResult();
$order->order_invoice_number = hikashop_encode($order, 'invoice');
$order->order_invoice_created = time(); // Add this line
$order->order_invoice_id = $order->order_id;
$order->order_invoice_number = $order->order_number;
$order->order_invoice_created = time(); // Add this line
It should fix your problem with the new option "invoice_reset_frequency".
Regards,