While checkipv4/6.dedyn.io works fine, the same services on .desec.io have a wrong certificate and even if ignoring that, the server will generate an empty reply.
Welcome to our community, and thanks for pointing this out!
To my knowledge, we never advertised checkipv4/6 under desec.io anywhere, and we actually do not run any services on those subdomains. The proper URLs for checking your IP reside under dedyn.io (https://checkip.dedyn.io/, https://checkipv4.dedyn.io/, and https://checkipv6.dedyn.io).
On hostnames that we don’t advertise on our website or in our docs, we do not guarantee any reasonable response, although one may argue that a certificate can’t be expected, and an empty reply is actually reasonable
If we mistakenly advertised those URLs under desec.io, please let us know and we’ll correct the error.