I am back. It took me a while to answer for several reasons. The one reason is that the twisting happened two more times. The last time 5 minutes ago, when going to the office…
The cause for the twist appears most likely to be the fact, that the rear wheel twists towards the frame from time to time. It seems that the wheel has some kind of freedom as the frame has a u-shaped scrap mark from the rubber of tire on the lower/horizontal pipe of the frame.
The question is how I can fix the movement of the wheel. I assume that some axle-parts and/or ball-bearings have to be exchanged?
However as I am a bit below the 100kg payload maximum of the Strida and the roads I have to take are not the best, I assume that I will have to switch to another bike 