sec.selfhost.de has no CNAME records.
Question and response
Question
dig sec.selfhost.de. NS
Answer
The response has no answer records.
Authority
selfhost.de. 2384 SOA pri.selfhost.de. hostmaster.selfhost.de. 1772386406 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0