I'm also having this problem trying to access
http://www.mmo-chamption.com from both Safari 3.1.2 and Firefox 3.0.1 on Mac OS X 10.5.5. This worked fine before, but not sure what changed (and when) that caused it not to work any more. I can access other sites just fine.
I'm set up with O2 Broadband in the UK, and am using a double-NAT network solution to get to the internet (Public -> O2 Router -> AirPort Extreme -> laptop) and I don't have problems with other sites.
Google search yields little help. I'm surprised Apple hasn't pin-pointed the problem. It's clearly with Mac OS X and some variable on the host site. I also have a Windows PC that can access the sites just fine on the same network. So I don't think it's my network. I have some complex port forwarding that works on my network, so it's design should not be a factor.
Edit: I use Leopard, own an iPhone 3G and have iTunes 8 and iPhone 2.1 software installed. Could this be the common denominator beyond 'Mac OS X' ?
Message was edited by: sildani