We found the rapeauto was set to 1/f instead of 1/f^4, we put it back to 1/f^4 and adjust the gain: 

INPUT ATTENUATON : 1.9

PIEZO GAIN: 4 

We measured open loop TF: UFG = 20 kHz, phase margin: 55deg.

It didn't improve the stability problem we have on cavity transmission.