There are a couple known causes to this:
- Your firewall is blocking connection attempts to the auth server. Add an exception for both TCP and UDP on all ports and and retry.
- Your antivirus is checking (and blocking) the connections to the auth server. Disable HTTP checking (sometimes known as "Link Scanner") or add an exception in your anti-virus for Tribes 2 (TCP and UDP on all ports) and retry. See this entry.
- Set your affinity (instructions)
- Your copy of the game is corrupted. Try reinstalling or redownloading (We suggest using the torrent).
- The auth server is currently down.
Additionally, this tool may help determine whether the problem is your computer or the auth server actually having downtime: http://tribes2.turkeh.net/status/