Apex domains cannot have CNAME records.
Question and response
Question
dig openit.de. NS
Answer
openit.de. 5 NS ns3.plusserver.com.
openit.de. 5 NS ns2.plusserver.com.
openit.de. 5 NS ns1.plusserver.com.
Authority
The response has no authority records.
Additional
ns1.plusserver.com. 5 A 62.138.233.183
ns2.plusserver.com. 5 A 62.138.201.123
ns3.plusserver.com. 5 A 18.158.102.90
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0