To get rid of the json parse error, which was a problem with OC 188.8.131.52 which happened when you tried to update an orders history, I have installed OC 184.108.40.206.
Now I get and Internal Server Error instead when trying to update and orders history.
My customers do not get an order confirm mail either...
I have Created an api-user an associated it with the shop.
The problem looks like it comes from this method call in catalog/order/api/order.php:
$this->model_checkout_order->addOrderHistory($order_id, $this->request->post['order_status_id'], $this->request->post['comment'], $this->request->post['notify'], $this->request->post['override']);
But I cannot find out where to find the file for $this->model_checkout_order
there is nothing in the error.log either