"An error occured while trying to access this service." saving changed to Cloud file

Suddenly got a problem with Reader today. I am on Windows 10, with all the updates.

I have a number of files already saved to document cloud. I need to add notes to them (ie, select text, right-click, select Add Note to Text), and periodically re-save. I have been using this process for about two years now with no problems. But today, every time I try to save a note I get "An error occured while trying to access this service." Same error when I try to save a new file to the cloud. 

I have tried restarting Reader; logging out and back in; running Repair; entirely deleting Reader and reinstalling; rebooting, etc, etc.

My Cloud-saved files all seem to open fine in Reader and via web browser, and I apparently can add notes to them by going into Adobe Cloud via Chrome and opening them in the browser.

Only thing that has changed at my end since yesterday is that the Windows feature update to v 21H2 installed last night.

I am baffled. What's up with this, anyone?? Thanks in advance for any help.

Regards,

Graham Scott