Thanks thanks thanks!!!
Now it worked. I´m not sure if it has to do with that thing: Two of my tubes had the same name. After renaming the 3rd one I was able to boole. (Not sure if this was the mistake or the fact that I´m in my MOI-Beginnings...)
The little bit more complex modelling concept I didn´t follow because it has geometrical things which do not fit to my geometrical aimes I intend to achieve...
All in all - very happy about the results and once more my deep respect for you, your kindness and patience and not at least for Michael and his MOI...and I can start printing 3D...
Cheers, Lara