I recently installed version 1.8.2 on my mac with 2gb ram and osx 10.5.7. The result was that many torrents that I've had running for weeks suddenly reported "tracker returned a 4xx message"
Restarting the system and my cable modem and router did not resolve this. This mac is in the router's DMZ (not behind firewall).
I reverted to version 1.7.6 of transmission and the i have not seen any tracker 4xx messages.