When I clicked on Launch in Anaconda Navigator today, I received the following error message
Sorry, this is not working properly! Our server encountered an unexpected condition that prevented it from fulfilling your request. Internal Server Error Failed to connect to Hub API at 'http://127.0.0.1:8081/jupyterhub/hub/api'. Is the Hub accessible at this URL (from host: green-nbjupyterhub12)? Make sure to set c.JupyterHub.hub_ip to an IP accessible to single-user servers if the servers are not on the same host as the Hub.
Yesterday the Jupyter Notebook was able to start.