Apex domains cannot have CNAME records.
Question and response
Question
dig optivo.de. NS
Answer
optivo.de. 1 NS a.ns.optivo.de.
optivo.de. 1 NS b.ns.optivo.de.
Authority
The response has no authority records.
Additional
a.ns.optivo.de. 1 A 193.169.180.254
b.ns.optivo.de. 1 A 193.169.181.254
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0