[2016.3] Juypter notebook hangs on cell "run"; post error

Hi all,

I have a very strange issue since I upgraded from (macOS) 2016.2 to 2016.3. I followed this guide (create python 3.5.2 virtual env, create notebook, etc.).  I try to run a very simple print("hello world") in a cell and execute the cell and I get: 

[I 11:20:58.924 NotebookApp] Serving notebooks from local directory: [redacted]
[I 11:20:58.924 NotebookApp] 0 active kernels 
[I 11:20:58.925 NotebookApp] The Jupyter Notebook is running at: http://127.0.0.1:8888/?token=a0f62fb8627d55b11f31e6bccceda380259977a81471d418
[I 11:20:58.925 NotebookApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
[W 11:20:58.963 NotebookApp] None
[W 11:20:58.964 NotebookApp] 403 POST /api/kernels (127.0.0.1) 1.77ms referer=None

I try executing the cell again and I get the same post error:

[W 11:23:07.717 NotebookApp] None
[W 11:23:07.717 NotebookApp] 403 POST /api/kernels (127.0.0.1) 0.95ms referer=None

If I connect to the jupyter server directly via browser I have no problem executing the notebook.

Any Ideas?

Thanks