Thank you for your help!
pylon:
But if there's too much messages coming at once from the mixer to the arduino, something goes wrong, the arduino stops responding...
What are too much messages? What's the serial output in this case?
It's some hexa codes, almost the same as the Arduino sends when it asks to move a fader. When I move a fader on the mixer (with my hand), the mixers sends midi messages like "B0 63 20 B0 62 17..." (for every "step" so this makes quickly 50-100 bytes for one fader move). I will copy/paste the output soon.
pylon:
A "software reset":resetFunc(); //call resetdoes not reset the W5100, so you should better implement a correct error handling but to simply start over. I'm unsure if this even does what you expect on the Due.
I agree with you, this code is not "beautiful", more like a "hack". Better error handling is the good way to go 
But... the Arduino seems freezed and I don't know where/how I could detect it 
I had one more idea yesterdy: if I use my "DUE", I can create a second loop "multi-thread" that could verify if the main loop is dead... and "reboot/reset" if necessary...