ikegami's output shows Content-Length: 0, so the web server is perfectly aware it is serving nothing and seems happy with that result (thus 200 OK HTTP status). So what program is generating the content for the / URI? Does it log anywhere?

This also might not be happening on all servers in the cluster. Notice that BrowserUK's output shows a different version of Apache, and, more importantly, the default Content-Type is text/html but on ikegami's it is text/plain (even though it is 0 bytes). This is another hint that the program which generates the content for / URI is having trouble because it isn't changing the Content-Type away from Apache's (probably) default of text/plain.