Token authentication is enabled. You need to open the notebook server with its first-time login token in the URL, or enable a password in order to gain access. The command:
jupyter notebook list
will show you the URLs of running servers with their tokens, which you can copy and paste into your browser. For example:
Currently running servers: http://localhost:8888/?token=c8de56fa... :: /Users/you/notebooks
Or you can paste just the token value into the password field on this page.
Cookies are required for authenticated access to notebooks.