Still running Apache 2.0.x as well; looks like it could just be a server going down/offline due to bitrot? There were a lot of great discussions in that wiki, hope it comes back up (though it's in the wayback machine, at least).
Archive.org to the rescue:<p><a href="https://web.archive.org/web/20160925063638/http://c2.com/cgi/wiki?WelcomeVisitors" rel="nofollow">https://web.archive.org/web/20160925063638/http://c2.com/cgi...</a>
Since there's still a link to it on the home page (<a href="http://www.c2.com" rel="nofollow">http://www.c2.com</a>), this may be a temporary situation.
I was trained as a developer under some of those early masters in Agile/XP. Including frequent references to that wiki. I sure hope we can reconstruct it and get it back online as soon as possible!