As of lately, I’m getting the following errors while nixos-rebuild switching:
error: cannot connect to daemon at '/nix/var/nix/daemon-socket/socket': Connection refused
Activation script snippet 'nix' failed (1)
What does it mean?
I’ve just seen this too, seems quite worrying 
EDIT: See this post, originating from this issue.
TL;DR: the issue is still unsolved