Yeah, unfortunately the ACME module is kind of weird on 20.09, I got bitten by this as well.
The following workaround solved the problem for me: Restructure acme module by m1cr0man · Pull Request #91121 · NixOS/nixpkgs · GitHub
Also related: ACME fails with JWS verification error · Issue #101445 · NixOS/nixpkgs · GitHub