Invalid request state: oauth2 flask
This can happen when the cookie session grows to large and the new CSRF token can't be saved to the session. Can you check the cookie sizes in your responses?
We fixed part of this problem with oauth2client 2.0.2, but if you're still experiencing it with that version I'd like to get some more info.
We do highly recommend using a database-backed session such as memcache/redis instead of cookies.