Do you get the message when starting the launcher or when pressing "Play cRPG" in the launcher?
I remember getting such a message when I would click "Play cRPG".
The reason for my error was that my external hard drive magically switched the partition from G to H, so that my cRPG obviously had no valid path anymore.
I launched Warband once (my Warband is Steam) and then it automatically readjusted the path and it worked fine again.