Apex domains cannot have CNAME records.
Question and response
Question
dig lc-dataroom.de. NS
Answer
lc-dataroom.de. 5 NS ns3.luckycloud.de.
lc-dataroom.de. 5 NS ns2.luckycloud.de.
lc-dataroom.de. 5 NS ns1.luckycloud.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0