Save to Document Cloud isn’t working

The "Save to Document Cloud" function in Acrobat Reader has suddenly stopped working.

I get the error message, "Unable to save the file to Document Cloud.  A copy has been saved to your device."  Initially it gives me the usual green message, "Saved to Document Cloud," but a few seconds later it switches. 

I'm using a 2020 iPad Air, running the latest version of iOS.  I've checked my iPhone, and the issue occurs there too, so it appears to be a problem with the app.
I've never had a problem before.  
Both Reader & iOS last updated on the 27 July, this problem first appeared four days later.   I successfully saved files to Document Cloud during those four days, so it doesn't seem to be related to the updates.  All files seem to be affected, not just ones downloaded today - I've tried saving files I've previously saved successfully, with no luck. 
I've tried everything I can think of - logged out of the app & in again, reset the iPad, checked the settings for iCloud & Reader - nothing has worked. 
I've also deleted & reinstalled the app, made no difference. 
Any ideas would be very much appreciated!