Apex domains cannot have CNAME records.
Question and response
Question
dig 207.235.167.clients.your-server.de. NS
Answer
207.235.167.clients.your-server.de. 5 NS ns1.your-server.de.
207.235.167.clients.your-server.de. 5 NS ns.second-ns.com.
207.235.167.clients.your-server.de. 5 NS ns3.second-ns.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0