Hi,
Thank you for your fast response.
It didnt come to my mind that it could be about Kaleido. From your suggestion I searched the forum about Kaleido on Windows, and it seems now to me to be very probable that this is causing the issue. Statik image export using kaleido - 
Plotly Python - Plotly Community Forum
My Kaleido version is 0.2.1. While writing this reply I tried to install version 0.1.0post as one commentator suggested and see if that works, and it worked! 
I just want to learn and observe what attributes control every detail of figures and how I could customize them even futher.