we use the following HTTP GET request to get group's info:
https://graph.microsoft.com/v1.0/groups/{groupId}?$select=visibility,unseencount
Until today (18 Sep 2018) it worked, but today it stopped working. Now Postman returns 403 Forbidden: ErrorAccessDenied (message: Access is denied. Check credentials and try again).
When unseencount is removed from url data is returned successfully with the same bearer token:
https://graph.microsoft.com/v1.0/groups/{groupId}?$select=visibility
Also in our app which uses Sharepoint SPFx and HttpClient it currently returns 400 Bad request for the same url.