Changes since SOA version
IXFR is a zone transfer request that returns changes since a specific SOA version. Enter the SOA version to request changes since then.
Not an apex domain
The domain mail.kieferorthopaede-muelheim-zentrum.de. is not the root of its zone.
Instead, try zone transfer on kieferorthopaede-muelheim-zentrum.de .
Instead, try zone transfer on kieferorthopaede-muelheim-zentrum.de .