When up2date is configured not to use proxies, but port 80 traffic is transparently sent through a proxy at the isp, bad things can happen, because up2date doesn't specify in the http traffic what host the request is for (unless configured to use a manual proxy), and transparent proxies (like my isp's) can think the request is for themselves, and will return a 404 not found error. Perhaps up2date should, instead of sending a "POST /cgi-bin/server.cgi HTTP/1.0" request, send a "POST http://priority.redhat.com/cgi-bin/server.cgi HTTP/1.0" (for example) whether it is configured to use proxies or not?
the update agent from7.0 will shortly be released for 6.x, and it includes this feature.
Up2date is now sending the following to the server: "POST https://www.rhns.redhat.com/XMLRPC HTTP/1.0" So that should take care of the problems related in this posting. Closing out this issue.