OK, but the interface should work as well, shouldn't it?

BTW, I tried the manual way through the terminal, and that didn't quite work either, but at least it deleted the old partions, and then the interface installation completed.

The only error I got was the unfree software, but at least I can report that NixOS is now installed and running.

I managed to make some changes to the config file, tested one rebuild and rollback, and now I can see the two generations listed. Cool. I like it. Just the installation was frustrating compared to other distros.