Jupyter notebook: No connection to server because websocket connection fails

I had the same problem, and following this vvk2001github https://github.com/jupyter/notebook/issues/4399 fixed it for me.

Uninstall tornado 6 and reinstall tornado 5.

sudo pip3 uninstall tornado
sudo pip3 install tornado==5.1.1

@kaleo gave this answer as well to similar question from @Jason Nichols