Apex domains cannot have CNAME records.
Question and response
Question
dig sebhosting.de. NS
Answer
sebhosting.de. 2 NS hydrogen.ns.hetzner.com.
sebhosting.de. 2 NS helium.ns.hetzner.de.
sebhosting.de. 2 NS oxygen.ns.hetzner.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0