@e06c1f25 Started playing around with https://messwithdns.net/ (instead of going to sleep like a good human) and it seems CNAME records don't accept when the second level of a domain contains a hyphen…? This is a valid domain however: https://ese.fsr-i.de/ 

 @ae28eec1 thanks! I created an issue https://github.com/jvns/mess-with-dns-backend/issues/32 (though I'm very behind on the backlog...)