Thanks pianoleo.
I just found this thread:
And I did what you suggest, the thousands midi channels are gone.
But why ist so?
Does it mean one has to care and “clean” or reset each project?
You surely know that there are good reasons to recycle the same project.
This is the only way to keep master pages.
In the meantime my master pages set is quite complete including all kind of pages for cover, content, empty pages and so on and it is quite a lot of work to redo this master pages set from scratch.