Apex domains cannot have CNAME records.
Question and response
Question
dig ns.rackspeed-cloud.de. NS
Answer
ns.rackspeed-cloud.de. 5 NS ns.rackspeed-cloud.de.
ns.rackspeed-cloud.de. 5 NS ns.rackspeed-cloud.eu.
ns.rackspeed-cloud.de. 5 NS ns.rackspeed-cloud.com.
Authority
The response has no authority records.
Additional
ns.rackspeed-cloud.de. 5 A 91.99.197.63
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0