Apex domains cannot have CNAME records.
Question and response
Question
dig internet24server.de. NS
Answer
internet24server.de. 5 NS ns.inwx.de.
internet24server.de. 5 NS ns3.inwx.eu.
internet24server.de. 5 NS ns2.inwx.de.
Authority
The response has no authority records.
Additional
ns3.inwx.eu. 5 A 45.87.158.53
ns3.inwx.eu. 5 AAAA 2a02:d500:0:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0