Apex domains cannot have CNAME records.
Question and response
Question
dig proserver.punkt.de. NS
Answer
proserver.punkt.de. 5 NS nameserver2.pluspunkthosting.net.
proserver.punkt.de. 5 NS nameserver1.pluspunkthosting.de.
proserver.punkt.de. 5 NS nameserver3.pluspunkthosting.de.
Authority
The response has no authority records.
Additional
nameserver1.pluspunkthosting.de. 5 A 217.29.32.132
nameserver2.pluspunkthosting.net. 5 A 217.29.32.136
nameserver3.pluspunkthosting.de. 5 A 185.142.213.130
nameserver1.pluspunkthosting.de. 5 AAAA 2a00:b580:8000:14:64a5:3288:5fef:e16d
nameserver2.pluspunkthosting.net. 5 AAAA 2a00:b580:8000:14:1052:7547:8d23:94cf
nameserver3.pluspunkthosting.de. 5 AAAA 2001:1680:101:c3e:4c8:d92e:7bca:7fca
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0