Apex domains cannot have CNAME records.
Question and response
Question
dig htpippbx.de. NS
Answer
htpippbx.de. 4 NS ns.htp-tel.de.
htpippbx.de. 4 NS ns1.htp-tel.de.
htpippbx.de. 4 NS ns2.htp-tel.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0