My mirror was some sort of automatic high speed cut-off. Everytime I reached 120 kph it flipped against the vent window due to the wind load. I tried several times to tighten the nut to achieve higher clamp forces, but the material is so weak, I stripped the thread!
Luckily, we have a huge "helicoil" repair set in our company, which I borrowed today.
This is the problem: