Hi, @FlorianWerndl, would you please first delete the inactive DNS zone here?
There is more information about why that should be deleted here:
There is a second issue. The primary custom domain for this site is www.bausatzbuch.de and not bausatzbuch.de. Because of this the branch subdomain must be branch-name.www.bausatzbuch.de and not branch-name.bausatzbuch.de.
To resolve this second issue, either make the CNAME record for develop.www.bausatzbuch.de or make bausatzbuch.de the primary custom domain for the site (under Site Name > Settings > Domain management > Custom domains).
If there are any questions about this, please let us know.