This page (and the v6 and v5) http://www.princexml.com/doc/7.0/xinclude/ says that the namespace for XInclude is http://www.w3.org/2003/XInclude, but it should actually be http://www.w3.org/2001/XInclude instead.

If you go to the w3 URL, you'll see that the 2003 says "This namespace name is deprecated and must not be used."

Thanks, this should be fixed now.