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

[gary1193]

I have the same issue, anyone got an answer yet.

[6502c]

I'm running into the same problem during my first attempt to set this up.
I also have a native IPv6 connection via my provider. You think this is related?

[Horgul123]

I switched to nginx in a Proxmox VM, now it's working