The odd thing is that it was working a few days ago. Even uploaded a web scene to arcGIS online, straight from CityEngine without trouble. Not sure what changed, but I doubt it was a change in proxy or anything of the kind. And no issues accessing arcGIS online from arcGIS.
So it's a CityEngine issue.
I don't know if it's a coincidence but it seems to have changed after I exported for the first time a webscene to ZIP (right click .3ws file in catalog-> share as-> save package to file), rather than to arcGIS online.
In meantime, I've been exporting the scene as zip files and uploading them to arcGIS online in a browser. Which works. But still puzzled as to why it's disabled from within CE.
cheers,
Tom