Apex domains cannot have CNAME records.
Question and response
Question
dig co3learn.de. NS
Answer
co3learn.de. 4 NS dns3.gwdg.de.
co3learn.de. 4 NS dns1.gwdg.de.
co3learn.de. 4 NS dns2.gwdg.de.
Authority
The response has no authority records.
Additional
dns1.gwdg.de. 4 A 134.76.33.41
dns2.gwdg.de. 4 A 134.76.33.42
dns3.gwdg.de. 4 A 193.175.210.140
dns1.gwdg.de. 4 AAAA 2a06:93c7:e1:0:0:0:0:1
dns2.gwdg.de. 4 AAAA 2a06:93c7:e2:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0