Currently, there is a bug how xrdp is packaged on NixOS. See xrdp: use patched xrdp sources when building xorgxrdp by chvp · Pull Request #250189 · NixOS/nixpkgs · GitHub for a fix. If you would like to try it, add this to your config
services.xrdp.package = let
patchedNixpkgs = pkgs.fetchFromGitHub {
owner = "chvp";
repo = "nixpkgs";
rev = "56d50b35b99e6c624933ad1d267aca23b49ae79c";
sha256 = "aam37J/wYN8wynNHBUgUVzbdm6wVXP+uq9CaOq1gscg=";
};
in pkgs.callPackage
"${patchedNixpkgs}/pkgs/applications/networking/remote/xrdp" { };