I've tried to losen the prop blades a little bit as adviced. No visible result, jello still present.
Today I'll try to tighten the motor dampers, maybe add washers...
If it were my problem this is what I would do.
First, log flight data for each ESC (if you are using a DJI controller, get an iOSD Mk II). Maybe you can see where one or two arms are either running at odd power levels, oscillating, etc.
Second, just check everything including balance. Be completely sure that the rig with everything installed is as perfectly balanced as possible.
Make sure the gimbal is balanced as well, and check yaw balance in addition to pitch and roll (pick up the copter, turn it 90% so the gimbal yaw axis is a balance point, and make sure that the gimbal doesn't want to rotate one way or the other around that yaw axis).
Check screws and look for wobble or movement on each arm, motor mount, prop mount, etc.
Check the gimbal attachment points. Look for loose screws, binding or tightness that shouldn't be there, etc. The rubber dampers should flex and absorb movement.
Check the camera to gimbal mount. The camera should be solidly and securely mounted.
Fly it. Check the logs. Compare ESC/motor output. Look for an outlier. If you find one swap it with an opposite arm. Fly again. See if the problem follows the bad arm. If it does, replace it or tear it down and build it back checking everything on it. If the problem stays in the position check the contacts at that position (electrical and mechanical).
