OK. I've made some progress. I completely rebuilt my .env file and changed the domains. Now I'm faced with the following error:
Connection refused [tcp://127.0.0.1:6379]
The error happens at:
pixelfed-manage migrate:fresh
while trying to rewrite
"add_software_column_to_instance_table"
The whole log is also in the NixOS Discourse:
https://discourse.nixos.org/t/pixelfed-on-nixos-server/49791/3