Thanks for reporting Matteo. Could you share with us the notebook? One easy way to share is to upload it to gist.github.com, and then copy the link of that gist to nbviewer.com.
What version of Plotly are you running?
Try running:
plotly.__version__```
You may have to run `pip install plotly --upgrade` if you're not using **version 1.9.0** or higher as mentioned: https://plot.ly/python/offline/