Apex domains cannot have CNAME records.
Question and response
Question
dig 3uu.de. NS
Answer
3uu.de. 5 NS ns.opst.de.
3uu.de. 5 NS ns2.opst.de.
Authority
The response has no authority records.
Additional
ns2.opst.de. 5 A 185.95.98.53
ns2.opst.de. 5 AAAA 2a00:1c3e:98:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0