Apex domains cannot have CNAME records.
Question and response
Question
dig werk21system.de. NS
Answer
werk21system.de. 5 NS ns3.werk21system.de.
werk21system.de. 5 NS ns2.werk21system.de.
werk21system.de. 5 NS ns1.werk21system.de.
Authority
The response has no authority records.
Additional
ns1.werk21system.de. 5 A 149.233.9.125
ns2.werk21system.de. 5 A 49.13.240.49
ns3.werk21system.de. 5 A 91.216.245.193
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0