Neither worked and no, not an old out of date machine.
As it was a local machine and fairly powerful I was able to copy across the config and build locally.
Time to look into Colmena I think.
I did test and it should work. Unless you provide logs and more information (like the commit of nixpkgs you’re using) I can’t really help you.
As I mentioned I’ve resolved with a local build in this instance so nothing more to test.