Thanks for your reply,
but what about this:
https://www.mauwen.com/docs/protect-wordpress-uploads/hooks-filters/
It says “This filter will change a protected file to the right URL /download/file/123/. This could be used if you have linked somewhere in your post, page or product. ”
Sounds like it is replaceing the old link from ../private/thumb/… to ../download/file/.. and thats exactly what ist necessary for me.