Thanks,
I did put a test file into the .well-known/acme-challenge/ path using ftp first and already saw, that no matter what I stick into the acme-challenge/ folder, my webserver always returns a blank page. (even if I enter an url which does not exist, the webserver does return a blank page, not a 404)
If I rename the acme-challenge folder it starts working. A acme-challenge folder somewhere on my Webserver also works fine. As soon as I create the acme-challenge folder inside the .well-known folder, the webserver returns a blank page.
Is this something my hosting company needs to look into ?