Hey, thanks all for the answers !
I tried the solution without the SerialEvent routine by putting all in the main loop but I am having the same problem, so it's seems that it's a baudrate problem.
Before I used an arduino nano and the 115200 baudrate was working great, and I have also used this baudrate ok for some time with the arduino pro mini without any problem, so that's why I don't understand what's going on...