API Problem setting the fulfilment_status

[RoliH]

We're using https://github.com/phpclassic/php-shopify as our framework to communicate with the shop

and we're getting the orders all fine. Just when wanting to switch the fulfillment_status to "fulfilled" by using this code

$shopify = new PHPShopify\ShopifySDK($config);
$updateInfo = array (
    "fulfillment_status" => "fulfilled",
);
$orderID = '3067744911535';
$shopify->Order($orderID)->put($updateInfo);

it doesn't change the status of Order ID 3067744911535 and no error return message.

This next try doesn't work either, but it returns an error

POST (SHOPURL)/admin/api/2020-10/orders/3067744911535/fulfillments/2878406688943/complete.json

{
    "errors": "An error occurred, please try again"
}

2878406688943 is the fulfillment id, that we're getting from /admin/api/2020-10/orders/3067744911535/fulfillments.json

Another attempt was

PUT (SHOPURL)/admin/api/2020-10/orders/3067604107439.json

with this content

{
  "order": {
    "id": 3067604107439,
    "fulfillment_status": "fulfilled"
  }
}

This does not return an error but also doesn't change the fulfillment_status.

Any chance someone can help us?

Thanks,

RoliH