Thanks for your reply. I purged the cache, logged out and back again, checked firewall, etc... nothing changed.
I am using Indesign v 17.2 on a Mac Mini (iOS version 10.15.30).
In the meantime, it appears that on the day I stated getting the error, we had cancelled the automatic renewall of the actual plan. But the plan is still active until July 2022. This seems to be linked to the problem we are now having with Publish Online.