Apex domains cannot have CNAME records.
Question and response
Question
dig isprit2.de. NS
Answer
isprit2.de. 3 NS ns1.isprit2.de.
isprit2.de. 3 NS ns0.isprit2.de.
Authority
The response has no authority records.
Additional
ns0.isprit2.de. 3 A 185.154.148.8
ns1.isprit2.de. 3 A 116.202.182.51
ns0.isprit2.de. 3 AAAA 2a14:3b00:1:7140:0:0:0:30
ns1.isprit2.de. 3 AAAA 2a01:4f8:1c1c:27dd:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0