On the desktop version only, the problem no longer persists, thankfully. (I'm using Evernote 10.5.7 on my Dell laptop running Windows 10.)
However, the problem persists on the mobile version. I still get the "Decryption failed. (null)" error message. I'm using Evernote 10.2 on an iPhone 12 Pro Max running iOS 14.2.1. However, I see that iOS 14.3 is available as an update for installation, so it's possible that the iOS update will correct for the Evernote MS Word encryption issue. I will try to remember to update my iOS tonight.