Parse invalid session token (Code: 209, Version: 1.7.1)
Delete the app from the simulator or your phone, then rebuild and re-run the app. Sometimes an old token gets cached. I just solved this problem for my app.
Check out this page: https://parse.com/tutorials/session-migration-tutorial
Most likely your app was using legacy session tokens. You can migrate to revocable session tokens by adding this to your app delegate right after you set the app id and client key.
[PFUser enableRevocableSessionInBackground];Parse's website has a section on handling this error that might be of more help to you: https://www.parse.com/docs/ios_guide#sessions-handleerror/iOS
Some of the answers posted were temporary fixes - however the problem could reoccur later if a user logged out and tried to log back in again - I think this was due to me allowing anonymous users also - the only way I could clear it is to call [pfuser logout] prior to log in