Apex domains cannot have CNAME records.
Question and response
Question
dig kunden.de. NS
Answer
kunden.de. 4 NS ns.kunden.de.
kunden.de. 4 NS theopolis.avency.de.
kunden.de. 4 NS sec.avency.de.
Authority
The response has no authority records.
Additional
ns.kunden.de. 4 A 20.86.163.145
sec.avency.de. 4 A 195.60.126.189
theopolis.avency.de. 4 A 195.60.102.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0