Apex domains cannot have CNAME records.
Question and response
Question
dig jd1.de. NS
Answer
jd1.de. 4 NS ns.kellerbu.de.
jd1.de. 4 NS ns.jd1.de.
Authority
The response has no authority records.
Additional
ns.jd1.de. 4 A 185.21.224.3
ns.jd1.de. 4 AAAA 2001:4c08:2003:6:10:10:21:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0