Thanks for your reply. I was going "Crazy" actually for the last three days. I have figured out the solution you mentioned here. Seems a solution to go through a situation, but actually...
I have found out a better implementation in Python and JSON for Moodle 2 Web services @ MooConsole: http://sites.google.com/site/mooconsole/
Unfortunatly I'm not fluent @Python or JSON, so I'm thinking to go back to XML RPC method for now.
Anyway, Best wishes with your project.