Hi Sivert,
Welcome!
I suppose the engine starts allright when cold?
If yes, then, as you allready suggest, something is wrong with one of the engine sensors. In this case the ignition point sensor (OT in german). However, when changing this sensor, you should also change the other one right next to it, which is the RPM or flywheel sensor. The reason is, that both are not very easy to reach, because they sit side by side in a "hole" between the engine block and the muffler.
I think, but am not sure, that there could also be a malfunctioning due to a lack of grounding. Each sensor will set you back some Euro 100.
regards,
Eric
