Apex domains cannot have CNAME records.
Question and response
Question
dig enova-steuern.de. NS
Answer
enova-steuern.de. 5 NS ns3.kas-dns-server.de.
enova-steuern.de. 5 NS ns6.kasserver.com.
enova-steuern.de. 5 NS ns5.kasserver.com.
enova-steuern.de. 5 NS a-dns-service.de.
Authority
The response has no authority records.
Additional
ns5.kasserver.com. 5 A 85.13.128.3
ns6.kasserver.com. 5 A 85.13.159.101
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0