Apex domains cannot have CNAME records.
Question and response
Question
dig kundenserver.de. NS
Answer
kundenserver.de. 4 NS ns-1and1.ui-dns.biz.
kundenserver.de. 4 NS ns-1and1.ui-dns.de.
kundenserver.de. 4 NS ns-1and1.ui-dns.org.
kundenserver.de. 4 NS ns-1and1.ui-dns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0