Apex domains cannot have CNAME records.
Question and response
Question
dig ewe-ip-backbone.de. NS
Answer
ewe-ip-backbone.de. 5 NS ans1.ewetel.net.
ewe-ip-backbone.de. 5 NS ans2.ewetel.de.
ewe-ip-backbone.de. 5 NS ans3.ewetel.net.
ewe-ip-backbone.de. 5 NS ans0.ewetel.de.
Authority
The response has no authority records.
Additional
ans0.ewetel.de. 5 A 80.228.127.14
ans1.ewetel.net. 5 A 213.170.187.14
ans2.ewetel.de. 5 A 212.6.121.254
ans3.ewetel.net. 5 A 85.16.255.14
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0