From logcat after adding my new leaderboard with name Highscores:
08-17 18:03:32.421 27888-29192/? E/Volley﹕ [14622] BasicNetwork.performRequest: Unexpected response code 400 for https://www.googleapis.com/games/v1/leaderboards/Highscores?language=en_GB
08-17 18:03:32.425 27888-32317/? E/LeaderboardAgent﹕ Unable to retrieve leaderboard Highscores
08-17 18:03:32.426 27888-32317/? E/LeaderboardAgent﹕ No instance found for leaderboard Highscores for PUBLIC and ALL_TIME