Vaadin TouchKit and Google App Engine = Session expired / Out of sync errors?

I've been developing with Vaadin, Vaadin TouchKit (for Mobile UI), and Google App Engine... and getting a lot of these messages:

Out of sync Something has caused us to be out of sync with the server.
Take note of any unsaved data, and click here to re-sync.

Also session expired messages......

Any ideas / solutions / workarounds ?

I'm considering switching to JSF 2.0 + PrimeFaces ...