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 static.15.187.203.116.clients.your-server.de is not the root of its zone.
Instead, try zone transfer on 203.116.clients.your-server.de .
Instead, try zone transfer on 203.116.clients.your-server.de .