Apex domains cannot have CNAME records.
Question and response
Question
dig creolineserver.de. NS
Answer
creolineserver.de. 5 NS ns1.creolineserver.com.
creolineserver.de. 5 NS ns4.creolineserver.com.
creolineserver.de. 5 NS ns3.creolineserver.com.
creolineserver.de. 5 NS ns2.creolineserver.com.
Authority
The response has no authority records.
Additional
ns1.creolineserver.com. 5 A 185.136.96.170
ns2.creolineserver.com. 5 A 185.136.97.170
ns3.creolineserver.com. 5 A 185.136.98.170
ns4.creolineserver.com. 5 A 185.136.99.170
ns1.creolineserver.com. 5 AAAA 2a06:fb00:1:0:0:0:1:170
ns2.creolineserver.com. 5 AAAA 2a06:fb00:1:0:0:0:2:170
ns3.creolineserver.com. 5 AAAA 2a06:fb00:1:0:0:0:3:170
ns4.creolineserver.com. 5 AAAA 2a06:fb00:1:0:0:0:4:170
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0