Apex domains cannot have CNAME records.
Question and response
Question
dig nervenklinik.at. NS
Answer
nervenklinik.at. 5 NS pns3.linuxpower.at.
nervenklinik.at. 5 NS pns2.linuxpower.at.
nervenklinik.at. 5 NS pns1.linuxpower.at.
nervenklinik.at. 5 NS pns4.linuxpower.at.
Authority
The response has no authority records.
Additional
pns1.linuxpower.at. 5 A 185.136.96.133
pns2.linuxpower.at. 5 A 185.136.97.133
pns3.linuxpower.at. 5 A 185.136.98.133
pns4.linuxpower.at. 5 A 185.136.99.133
pns1.linuxpower.at. 5 AAAA 2a06:fb00:1:0:0:0:1:133
pns2.linuxpower.at. 5 AAAA 2a06:fb00:1:0:0:0:2:133
pns3.linuxpower.at. 5 AAAA 2a06:fb00:1:0:0:0:3:133
pns4.linuxpower.at. 5 AAAA 2a06:fb00:1:0:0:0:4:133
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0