An "invalid session" error means the server-side session data is missing.
If you are using redis as session store, and you don't bother to copy it to the new server (because it's just transient data), then this can happen...
Or if you're using Mongodb only, but you only migrated the objects collection (which is all that is strictly required).
@crazycells Either way, if they see the message, clicking "ok" should automatically clear their local browser session. If not, that may be a bug.