With your analysis, hints and link to the specification, I was able to locate and fix the problem.

Thanks a lot for your help!

Thank you for your quick reply! Here is the entire XML example. Yes, we are using DocBook 5. <?xml version="1.0" encoding="UTF-8"?> <?oxygen RNGSchema="http://www.oasis-open.org/docbook/xml/5.0/rng/docbookxi.rng" type="xml"?> <sect1 xml:id="test" xml...

Adding the "xml" prefix to the "id" didn't help either:

With this code <sect3> <title>A Title</title> <para>Some text::</para> <para> <figure id="my_id"> <title>Figure title</title> <mediaobject> <imageobject> <imagedata fileref="images/blah.png" /> </imageobject> </mediaobject> </figure></para> </sect3> Oxygen XML Editor 13.2, build ...