Hmm, ok.
A couple of thoughts. Are you able to display non-plotly ipywidgets? For example, do you see a slider when you run this code?
import ipywidgets
ipywidgets.IntSlider()
Another thought: I think I remember hitting a 404 error at one point (across all ipywidgets) when had a an adblock extension update. So if you have an adblock extension make sure to whitelist localhost. Also try it in multiple browsers (at least chrome and firefox), and make sure the browser versions are up to date.
Sorry, I know this is a scattershot of suggestions. Thanks for sticking with it!
-Jon