Thank you @zipit for your comprehensive response, especially with the link to the coding guide. I guess it might be better to create the spline manually from the points which are connected by the edges without the usage of SendModellingCommand. It was good for prototyping functionality but as you mentioned, it is hard to debug. I appreciate you took the time (and I know it's a lot of convoluted code) to read through it. @r_gigante -> the work is currently on my side so you don't need to bother. I think I have enough information to do a second iteration and rewriting this from scratch.

Amazing! Thank you Manuel! It works with multiple objects, Cloners etc. Without your help I would not make it

The problem is solved. It was so trivial :D. I unchecked the plugins folder which I added in the preferences. My plugins folder was in the C4D directory, so as you described, it was loaded twice. Leaning for me: Never place a custom plugins folder in the C4D directory! Recommendation to the SDK team: Maybe you could adjust the error message in the console, which mentions that plugin folders should not be in the C4D directory. I would assume that many users/ devs will place the folder there, because of a habit from older versions. Thank you @r_gigante and @zipit for your quick response!