Turns out it had expired. You have to be very quick using the authorization code. Even waiting a minute seems to be too long.

I'm trying to go through the OAuth2 flow as described here https://marketing.adobe.com/developer/documentation/authentication-1/auth-web-server-1When I get to the final step of exchanging the authorization code for an access token I'm receiving a response with a http status code of 400 with an error...

I assume it applies to other solutions as well. I see that this method exists https://marketing.adobe.com/resources/help/en_US/aam/r_rest_api_user_get_self.html . Can I use this with an analytics oauth access token? 

Something like a /me endpoint that provides information about the authenticated user. I'd like to record the information so that when the account needs to reauthenticate we can show information to the user so they know which account they authenticated.