It responds for me, both at neth.firmaxy.de and at neth.firma-xy.de. It gives a 404 page:
However, an attempt to reach something in the /.well-known path (which is what the baked-in Let’s Encrypt support needs) fails with a strange error:
…which makes me wonder if it’s actually the Neth server that’s responding at all.
You might be better off following one of my guides for using DNS validation:
https://wiki.nethserver.org/doku.php?id=userguide:let_s_encrypt_for_internal_servers
https://wiki.nethserver.org/doku.php?id=userguide:let_s_encrypt_acme-dns