See https://github.com/jupyter/notebook/issues/2978 for a working config file (end of the thread), that is:

{
  "NotebookApp": {
    "password": "sha1:65e37f0d14de:5249cef964e888cf5b8f9cc99c8735e01c8e0f2f",
    "token": "10fd01baf5c7ff2db46cb937e42e1da8d19bd49023216e2b"
  }
}