Apex domains cannot have CNAME records.
Question and response
Question
dig artfiles.de. NS
Answer
artfiles.de. 1 NS auth3.artfiles.de.
artfiles.de. 1 NS auth1.artfiles.de.
artfiles.de. 1 NS auth2.artfiles.de.
Authority
The response has no authority records.
Additional
auth1.artfiles.de. 1 A 212.48.124.53
auth2.artfiles.de. 1 A 212.48.125.53
auth3.artfiles.de. 1 A 212.12.58.190
auth1.artfiles.de. 1 AAAA 2a00:1f78:af:a1:0:0:0:2
auth2.artfiles.de. 1 AAAA 2a00:1f78:af:a2:0:0:0:2
auth3.artfiles.de. 1 AAAA 2a00:ea8:100:6:0:0:a3:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0