Hmm... so they validate the keys via a server, probably the same one they run the website, but not the forums on? That'd be the only explanation I can think of.
("Start Search" seems to hang up for me, too. Everything was in order yesterday)
Edit: some threads on this:
http://forums.taleworlds.com/index.php/topic,107131.0.html
http://forums.taleworlds.com/index.php/topic,107086.0.html