PDFs suddenly won’t save to document cloud

The "Save to Document Cloud" function in Acrobat Reader has suddenly stopped working today. 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 five days ago, I have successfully saved files to Document Cloud since that time though, so it shouldn't 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. 
Any ideas would be very much appreciated!