The user defined callback URI is missing the GET method. This should be added as specified in SOL003 (2.3.1) Sec. 5.4.20.3.2 Description: The GET method allows the server to test the notification endpoint that is provided by the client, e.g. during subscription. Responses: 204 (no content) 4xx/5xx Error codes with Headers and body (ProblemDetails object)