Apex domains cannot have CNAME records.
Question and response
Question
dig second-ns.de. NS
Answer
second-ns.de. 4 NS ns1.your-server.de.
second-ns.de. 4 NS ns.second-ns.com.
second-ns.de. 4 NS ns3.second-ns.de.
Authority
The response has no authority records.
Additional
ns1.your-server.de. 2 A 213.133.100.102
ns3.second-ns.de. 4 A 193.47.99.4
ns1.your-server.de. 2 AAAA 2a01:4f8:0:1:0:0:5ddc:1
ns3.second-ns.de. 4 AAAA 2001:67c:192c:0:0:0:add:b3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0