The clue is this
Session contains objects whose class definition isn’t available.
Remember to require the classes for all objects kept in the session.
(Original exception: uninitialized constant ActionController::Flash::FlashHash [NameError])
The session contains serialised ruby objects and in this instance contains an object that rails couldn’t deserialize. If my memory is correct, ActionController::Flash::FlashHash was the name of the class that held the session values in older versions of rails - it looks like you might have accidentally upgraded rails. Clearing out sessions should fix this, although the upgrade might of course cause problems elsewhere.