Apex domains cannot have CNAME records.
Question and response
Question
dig mb-kfz-service.de. NS
Answer
mb-kfz-service.de. 5 NS ns1120.ui-dns.org.
mb-kfz-service.de. 5 NS ns1051.ui-dns.biz.
mb-kfz-service.de. 5 NS ns1021.ui-dns.com.
mb-kfz-service.de. 5 NS ns1031.ui-dns.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0