WEB_HOOK channel unavailable when trying to retrieve Google calendar events

Question

I'm trying to use Push notifications for Google Calendar

Callback endpoint is hosted on Heroku. appname.herokuapp.com is verified in Search Console and added to Google Console APIs & Services Allowed domains list.

Request

POST /calendar/v3/calendars/CALENDAR_ID/events/watch HTTP/1.1
Host: www.googleapis.com
Authorization: Bearer TOKEN
Content-Type: application/json
{
    "id":"1",
    "type": "web_hook",
    "address":"https://APPNAME.herokuapp.com/change"
}

Response

400 Bad Request
{
    "error": {
        "errors": [
            {
                "domain": "push",
                "reason": "channelUnknown",
                "message": "WEB_HOOK channel unavailable for: 
{address=https://APPNAME.herokuapp.com/change}"
            }
        ],
        "code": 400,
        "message": "WEB_HOOK channel unavailable for: {address=https://APPNAME.herokuapp.com/change}"
    }
}

What

WEB_HOOK channel unavailable

error means?

EDIT: same result with a top level domain which certificate's Subject matches exactly the domain name.

Answer 1

It appears that this was a temporary issue and is now fixed according to Google. (I also tested and seems to be working for me, now.)