It seems that as soon as I have something like

<extensions><system-message></system-message></extensions>

for example in <trk>...</trk> I get "javax.xml.stream.XMLStreamException: Trying to declare prefix "xmlns" (illegal as per NS 1.1 #4)".

See files below "WithoutExtension.txt" (can not upload ".gpx") when read and write again works. "WithExtension.txt" raises error.

GPX.Reader r = GPX.Reader.of(Version.V11, Mode.LENIENT);
GPX gpxWO = r.read(Path.of("WithoutExtension.txt"));
GPX.write(gpxWO, Path.of("rewriteWoE.gpx"));
GPX gpxW = r.read(Path.of("WithExtension.txt"));
GPX.write(gpxW, Path.of("rewriteWE.gpx")); //Error: javax.xml.stream.XMLStreamException: Trying to declare prefix "xmlns" (illegal as per NS 1.1 #4)

WithExtension.txt
WithoutExtension.txt

Thanks.