uninitialized constant ActionController::Flash::FlashHash [NameError])

The clue is this

ActionDispatch::Session::SessionRestoreError
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.

Fred

Thanks!
Seems your description is relevant, see answer to previous advice.