Thank you for the fast and helpful replies.
I conclude that as everything works just fine with Buwizz/normal which is supposed to be equivalent to PF/IR the problem is most likely the IR. Those do not work well in any combination IR R1/IR R2 vs. M/L/XL-motors.
And thinking of it, I build the Alfa Romeo 4C spider by Jeroen Ottens. This one worked just fine with IR. I guess I was lucky combining the fitting components.
I will check that.
I'm building these models to learn. I want to construct something on my own, so a simple model is not so helpful. I did some, such as the stuff published by Racing Brick and 42082, 42083, 42095, 42081. No offense, but with the instructions provided by LEGO these are very easy to build.