Hi Jörg, there are some irregularities in some areas of the model, like for example there's a little teeny tiny hook-like edge in this area here (from your first posted file):





These kinds of teeny tiny and also squiggly little edges are going to be problematic, basically those areas need to be rebuilt to be cleaner before you'll be able to get things joined up well between them.

I'll see if I can help you to make a tuned up version.

- Michael