I get the same error on a newly created virt-manager/qemu VM when home-manager is restarted from nixos-rebuilt --target-host my-vm ... with 63G free space
Just found a workaround. I had to add a user to nix.settings.trusted-users = [ "myuser"];
I can’t tell why this was necessary because I have several machines with similar configuration where this wasn’t necessary. I usually do nix operations using the admin user or root (sudo) which are already set as trusted users.