This is now ancient news, and Evernote apparently continues not to fix it. I am as ever using the latest client.
I managed to discover a work-around, however. None of the above tips seemed to affect anything.
But signing on to the web site version of Evernote, then adding a note there and saving it, followed by returning to the Windows 10 Evernote and doing Sync there, did the trick.
After the sync, I think the red banner was still present at a first, but then checking Tools>Account Info showed the monthly total had been reset, with only the new Note against it, and I could once again enter and sync Notes from Windows successfully.