Apex domains cannot have CNAME records.
Question and response
Question
dig datevnet.de. NS
Answer
datevnet.de. 5 NS ns02.datev.com.
datevnet.de. 5 NS ns01.datev.de.
Authority
The response has no authority records.
Additional
ns01.datev.de. 2 A 193.27.50.5
ns02.datev.com. 2 A 193.27.54.5
ns01.datev.de. 2 AAAA 2a00:e50:f155:a:0:0:0:5
ns02.datev.com. 2 AAAA 2a00:e50:f157:0:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0