I think I've found what caused my problem and you where true, the echo wasn't the problem.
I don't know why (maybe a copy/paste error between two projects I'm working on), but after my "echo" there was a "die();"... now it explains all ;-)
Sorry to have taken your time away. Rest assured I will report anything (real) I'll found about your ExtDirect, because I'll use it extensively the next weeks in a project I'm starting right now.
Thanks a lot to have taken some time to help me, it definately gave me some advice on my problem !