initially I thought that would be the problem, so I replaced the chain and rotor, these were too old anyway. I guess I can rule that out now, however the freehub is a good hint, will check it asap. Thanks!
I have this weird behaviour on my M600, it only happens when the chain is on one of the outer half cogs of the cassette and I am putting force on the motor, either by pedaling or by using the scooter mode switch and braking at the same time, as in the video below. I have disassembled the motor...