I recently tried to visit Codeplex and got a an error page with a Server 500 error. It did not take too long to figure out that there was a configuration problem on my firewall, ISA 2004. There were several proposed fixes but the one that worked for me I found on a Techarena forum and it said to either turn on or off the HTTP Compression filter. I turned it on and it worked.
I think I had turned off the compression filter in ISA 2004 SP1 days. According to Lazyadmin HTTP Compression started working in SP2 and he has recommendations for configuring it in his post, Enabling HTTP Compression in ISA 2004.