Problem is solved, to combine objects you need 2 selected objects and choose "boolean" -> "addition". My problem was that I had only one object left because I combined them without the booltool. I think that's not really an error that must be fixed.

Hi Karlay, your new website looks really nice and professional ;) After i've read that thing with "coustom insert brushes" I want to jump in again. In the last time I made new things in blender, so it seems so i can't stop learning 3D. I'm really excited to try out the new version. I also watched your speed-tutorial and that gave me ideas how to practice my skills. Thats exact the way i would use for my objects, that's why i have new hopes. Hope you are doing well, thanks for your continous development. Greetings Alex