Apex domains cannot have CNAME records.
Question and response
Question
dig sxb.network.ps-intern.de. NS
Answer
sxb.network.ps-intern.de. 5 NS ns1.plusserver.com.
sxb.network.ps-intern.de. 5 NS ns2.plusserver.com.
Authority
The response has no authority records.
Additional
ns1.plusserver.com. 5 A 62.138.233.183
ns2.plusserver.com. 5 A 62.138.201.123
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0