Webhook notifications are never sent ( BC14 CU16 on-premise)

Hello.

I have a Business Central version 14 with CU16 installed on premise.

My issue is that BC never sends notifications to my webhook endpoint. The subscription gets confirmed, but I never receive any notifications.

Currently, I only have a single webhook subscription on Items:

{
  "@odata.context": "https://bc14cu16.example.com:8112/BC_WS/api/beta/$metadata#subscriptions",
  "value": [
    {
      "@odata.etag": "W/\"JzQ0O1NxcUlZK215Z0lMTC9KM3ZROCtEQzFFZDVwRVJSbjRyZGpjZmt4L3QvaGM9MTswMDsn\"",
      "subscriptionId": "1805a065a9904b60a682574500704d0d",
      "notificationUrl": "https://outdated.eu.ngrok.io/api/v1/webhooks",
      "resource": "api/beta/companies(2afeb309-724c-4a5f-b628-136151b576e4)/items",
      "userId": "23b7e692-928d-4218-99da-e86b30558c7d",
      "lastModifiedDateTime": "2020-11-18T11:55:26Z",
      "clientState": "SADsA5rT2r1jT4wP",
      "expirationDateTime": "2020-11-21T11:55:26Z"
    }
  ]
}

I can renew and delete the subscription just fine. 

Whenever I update an item or more, nothing is sent to the specified notification URL.

I only receive notifications when I create or renew a subscription.

I've had issues with this previously - with CU04, webhooks didn't work. Spent a lot of time and money on a Microsoft Support ticket, just to learn that is was magically fixed somewhere between CU04 and CU10, since I got notifications while on CU10. 

Now I'm on CU16 and now I don't get any notifications any more. I don't know if it related to the previous issue.

Anyone know how to resolve this issue?