I just turn my iPad off then on again... basically 'reboot it' - works everytime.
It seems to happen after I've been creating a number of screen shots, pics, notes etc. in a row... I haven't counted yet to see if there's a correlation between the number of actions per 'session' and when the 'auth expired' kicks in... but I'm guessing that maybe it has something to do with that?
I really LOVE this app and it would be awesome if I didn't have to stop and reboot in the middle of following a train of thought during a brainstoring session