The ASCII and HTML responses are generated by accessing the server using the URL (a future version of the server may use a different design; this is true for version 3.4 and may be true for some later versions). If you are serving data behind a firewall which uses NAT for address traslation, the DNS lookup for the host name can fail.
Greg Miller from the USGS tracked down this problem and came up with a solution:
I am behind a firewall that uses NAT translation, so if it was relying on DNS to find the address, it would fail. I checked my host file and discovered that Red Hat maps the server name into the loopback address and not the IP address of the ethernet interface. I corrected the host file, and everything works fine.