The server does not have a DNS Entry. If the user reloads, it works fine. The DNS name is valid.
Most operating systems do only one DNS lookup at a time.
Using software like Proxy Server which does many DNS lookups can overwelm the operating system. Try turning the DNS caching on in the Proxy Server. This way the proxy will save DNS name and IP address for the time set on the configuration page. All lookups for that site will now be found in the cache and the operating system only has to lookup new sites. DNS caching can be found in the Admin Server under Server Preferences.