Apex domains cannot have CNAME records.
Question and response
Question
dig stundenzettel.online. NS
Answer
stundenzettel.online. 1 NS second-dns.netcup.net.
stundenzettel.online. 1 NS third-dns.netcup.net.
stundenzettel.online. 1 NS root-dns.netcup.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0