POST webhook returns 200 but empty array

[brickfox]

Dear community,

we are trying to register a webhook from a custom app.

This worked a few times in the past, but for a new customer, it is not.

We are POSTing to /admin/api/2023-10/webhooks.json with the payload

{
  "webhook": {
    "topic": "orders/create",
    "address": "https://***/webhook/orders-create",
    "format": "json"
  }
}

The API return status code 200 but an empty array of webhooks:

{
	"webhooks": []
}

When trying to GET the /admin/api/2023-10/webhooks.json endpoint, the same result is returned, so the webhook is not created.

How can I find the reason for the webhook not beeing created?

Kind regards

Niko