LetsEncrypt -> There is a server found at this domain [...] but [...]

[Horgul123]

I've set up a Nginx Server which is reachable via IPv6 (my provider doesn't assign public IPv4 addresses). I have a domain at a DynDNS provider.
Then I added my first proxy host and created a subdomain pointing to my Nginx server. When I enter this subdomain in a browser (mobile phone, not using Wifi) I reach the host I've configured.
So it looks like the domain (and the subdomains) is working.

Now I try to get a LetsEncrypt certificate. When I enter my domain in the the field in Nginx and test the server reachability I get

There is a server found at this domain but it returned an unexpected status code Invalid domain or IP. Is it the NPM server? Please make sure your domain points to the IP where your NPM instance is running.

I have no clue what I need to change :(

Tried to find a solution with Google, but it looks like I'm using the wrong search terms