Authentication error - solved Kept asking me to enable IMAP but it has never been switched off. Just realised it hasn't backed up since Jan 2015. Update: Since I upgraded my phone to Lollipop and now Marshmallow  the phone asked for reauthentication authorisation which only showed up on the notification bar. And I have missed that. Now that I have granted the permission  the app is backing up properly.
April 26 2016