Apex domains cannot have CNAME records.
Question and response
Question
dig _acme-challenge.loki.dt2.data.nubeo.io. NS
Answer
_acme-challenge.loki.dt2.data.nubeo.io. 2 NS diana.designplus.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0