[Nix-dev] What does "output is not allowed to refer to path ..." mean?

[Peter Simons]

Hi,

why does nixos-rebuild generate an error message such as the following one?

  output is not allowed to refer to path `/nix/store/qccjhn063cfv171rcaxvxh0yk96zf7l2-cryptsetup-1.4.1'
  cannot build derivation `/nix/store/svqdc9jf8qqn89lv93pxqwgky00cylj2-stage-1-init.sh.drv': 1 dependencies couldn't be built
  cannot build derivation `/nix/store/k9lgzyb92qq3hjy1bqxxf1mmjqbsgxpr-initrd.drv': 1 dependencies couldn't be built
  killing process 7263
  cannot build derivation `/nix/store/xyjdb6yvxi32lkf0amm84i50742y6rx3-system.drv': 1 dependencies couldn't be built
  error: build of `/nix/store/xyjdb6yvxi32lkf0amm84i50742y6rx3-system.drv' failed

What does that error mean?

Take care,
Peter