What cancels the permit for use of a motorized bicycle?

You answered correctly. Making technical changes like removing the silencer or modifying the gear ratio is not permitted because these changes can increase noise or speed, which breaks the approval conditions for a motorized bicycle. Adding a second rear-view mirror, however, does not affect these key approval conditions, so it does not cancel the permit. In German traffic law, any unauthorized technical modification that alters the performance, noise, or emissions of a vehicle will usually result in the loss of its operating permit. Always keep your vehicle in its approved technical state to remain compliant.