I actually should now this but i don't anymore
How is the dtd attached to the xml document
<!DOCTYPE rootNodeName SYSTEM "file path">
In the old day's of SGML this should have been a valid path to a system resource. So http stuff never would have worked. Maybe XML changed this but i do not know for sure.
Just a thought
You can find all DTD's in /docs/dtd directory
I know, but it would be easier if the HTTP system address worked - then IDEs could download the DTD automatically from the web without the user taking any special action.