AdobeSign - Validation for webhook url failed

I’m trying to register a webhook using the AdobeSign API (v6) !. Upon sending the POST I get:

{

    "code": "INVALID_WEBHOOK_URL",

    "message": "Validation for webhook url failed. Please make sure that the webhook url responds with a valid HTTP response code and a valid client id."

}

The body I sent is:

{

    "applicationName": "Test application",

    "name": "Somename - 5c0bc93d-b955-4451-bbc1-3b7e92c1cd6c",

    "state": "ACTIVE",

    "scope": "ACCOUNT",

    "webhookSubscriptionEvents": [  "AGREEMENT_ACTION_COMPLETED"  ],

    "webhookUrlInfo": {

        "url": "https://jan.X.com:3443/api/v1/flows/callback/c8b76d29-93e7-4f25-915e-a8ab600e93d5"

    }

}

Sending a GET to the url, containing the header “X-AdobeSign-ClientId: Some rnd id” it returns {  "xAdobeSignClientId": "Some rnd id"}. Sending a GET to this url using my phone over 4G also returns this. It takes about 30 seconds for AdobeSign to return the 400 BAD REQUEST.

Thanks