After regenerating our RELAX NG schema, Oxygen is giving us the above error on validating our TEI files.
I can't say I understand completely what is going on, but oXygen points to the macro.anyXML, that is included in the schema (in our case) because of the element xenoData. After removing that element from the schema, my problem goes away. (It also goes away when I tell oXygen not to do ID/IDREF checking).
But this is only a workaround of course. Can somebody explain what is going on?