I'm having this same problem, actually. Played countless Napeolonic Wars LAN battles in the past and now have not been able to play over LAN for a week. Every time I try to join a game (over LAN and if we try to host online), it says, "Invalid Authorization Key." All serial keys are different, private IP addresses are different, I have uninstalled and reinstalled, and unblocked all firewalls. Nothing has changed since the last time I played (i.e. no new computers, service providers, locations, etc.). I've got the game on Steam and both vanilla Warband and Fire & Sword can be played over LAN with no issue.