Apex domains cannot have CNAME records.
Question and response
Question
dig plutex.de. NS
Answer
plutex.de. 3 NS ns2.plutex.de.
plutex.de. 3 NS ns1.plutex.de.
plutex.de. 3 NS ns3.plutex.de.
Authority
The response has no authority records.
Additional
ns1.plutex.de. 3 A 91.202.40.53
ns2.plutex.de. 3 A 91.202.41.53
ns3.plutex.de. 3 A 116.203.36.144
ns1.plutex.de. 3 AAAA 2a02:16d0:0:1:0:0:0:53
ns2.plutex.de. 3 AAAA 2a02:16d0:0:1:0:0:1:53
ns3.plutex.de. 3 AAAA 2a01:4f8:c2c:df5b:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0