I see that this is related to the deprecation of the ACMEv1 API. The interesting thing to see for me was, that I have other let’s encrypt certificates on the same host already, so I thought a new account wouldn’t be used. NixOS seems to request a new account anyway and therefore getting a certificate for a new domain on this host doesn’t work at the moment.
The client used (simp_le-client-0.9.0) is rather old. However, it seems it uses the python ACME client library under the hood. So it probably does support ACMEv2 already.
It also seems simp_le has the --server option. So here are two options:
upgrade simp_le-client to a version ≥0.15.0 (from that version onwards, Let's Encrypt ACMEv2 server was the default ACME server)
add a --server option with LE's ACMEv2 endpoint as value.
Thanks for the comments. I got it working now. Actually NixOS already did upgrade simp_le from 0.9.0 to 0.16.0 already on (/not before) 2019-10-17. But after updating my Nix channel with nix-channel --update (something similar to apt-get update) it updated simp_le and I got the certificate.
19.03 is already end of support since yesterday. So probably isn’t no big problem, that this isn’t updated.
I didn’t notice that I haven’t updated my channel the last two weeks, or actually I thought I had. This server I had the problem with is managed remotely so I don’t have to do these updates there manually (normally).
So I guess there is no real problem, it my own fault.