Apex domains cannot have CNAME records.
Question and response
Question
dig kundencontroller.de. NS
Answer
kundencontroller.de. 5 NS ns1.kundencontroller.de.
kundencontroller.de. 5 NS ns3.kundencontroller.de.
kundencontroller.de. 5 NS k1.kundencontroller.de.
kundencontroller.de. 5 NS ns2.kundencontroller.de.
Authority
The response has no authority records.
Additional
k1.kundencontroller.de. 5 A 85.31.185.124
ns1.kundencontroller.de. 5 A 85.31.185.126
ns2.kundencontroller.de. 5 A 85.31.184.6
ns3.kundencontroller.de. 5 A 85.31.184.7
k1.kundencontroller.de. 5 AAAA 2a02:180:ffff:1:0:0:551f:b97c
ns1.kundencontroller.de. 5 AAAA 2a02:180:ffff:1:0:0:551f:b97e
ns2.kundencontroller.de. 5 AAAA 2a02:180:ffff:1:0:0:551f:b86
ns3.kundencontroller.de. 5 AAAA 2a02:180:ffff:1:0:0:551f:b87
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0