Apex domains cannot have CNAME records.
Question and response
Question
dig qualityserver.de. NS
Answer
qualityserver.de. 4 NS ns1.domainkunden.de.
qualityserver.de. 4 NS ns2.domainkunden.de.
Authority
The response has no authority records.
Additional
ns1.domainkunden.de. 4 A 195.234.228.7
ns2.domainkunden.de. 4 A 20.113.63.118
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0