I have just installed Jupyter over pip (Python version is 3.7.2) in Windows 10, started the jupyter server by calling jupyter notebook, created a new notebook w
jupyter notebook
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