Hi!
you can check the xml by "hand", just open it with an texteditor.
OK, you will not recognize the modell, but you can find suspicious values (very high, or maybe to few faces etc.)
Also you should use the newest exporter for blender.
You can download it on
http://http://www.titusgames.dei also get some error-messages, but since the modell is fine i do't care...
you could also try to convert the g3d back into xml and then you can open it with blender again (or you look ingame Glest, or by the Glestviewer)...
// edit:
oh, didn't read carefully enough, you already used the Glestviewer... but my advices are still the same.