OAuth2 Authorization Code Flow No Longer Works

Hi,

I'm using authorization code flow for authorization.

The URL that I'm using for a browser is:

https://authz.constantcontact.com/oauth2/default/v1/authorize?client_id={myClientID}&scope=contact_data+campaign_data+offline_access&response_type=code&state=yNU4YJIKZz&redirect_uri=http%3A%2F%2Flocalhost%3A31000%2Fccauth

The redirect uri matches what was set up in the application.

This used to work up until at least a few months ago.

Now, I get the following where it shows "Unable to sign in" with no error message:

And it remains in this state indefinitely.