A visitor attempts to authenticate with my application but isn't signed into Meetup. The Meetup signin form is shown with a link to sign up if the visitor doesn't have an account. If this route is taken, after creating an account and verifying their email, the visitor is taken through a series of questions to identify their interests and are then dropped on their Meetup personal page. No longer are they returned to the address specified in redirect_uri during the initial OAuth handshake.
If the visitor has a Meetup account and signs in or is already signed in then the process goes smoothly and the visitor is returned to our application.
The failure to do so after signing up for a new Meetup account is a new problem; new as far as it has worked fine for years until I just ran through some testing and identified it. Not sure exactly how long ago it stopped working however.
Mike