Thanks for looking. Not sure what´s the consequence of your statement...
But I remodeled it and I could stitch the elements now.
Thanks for your tipps and very nice help - Pilou and Michael!!!
Salut, MaLa

P.S.: Next project I´ll try to find out what the difference for stitching is using Boole or Connect Tool...

What method have you used for success your stitching ?

Have a look at the videotutorials from chippwalters.

This covers your problem nicely:
https://www.youtube.com/watch?v=2-5J6hLRDaI

Easy peasy.

Indent/outdent. Just loft the surface with the kink in it, instead of trying to add it later...


Top, straight surface, and curved, outdented...



Bottom, indented...