Hi again,
Indeed it is very tricky.
I found the cause of the exception, and it's specific to my configuration.
In blazeDS, I was using a custom BeanProxy that was registered against the Object class.
That's what was causing the exception.
I've registered that BeanProxy against another class and don't have the exception anymore.
But the XML layout of the response is still not as nice to read as the one from the tutorial in the demo.
It looks like this :