Thanks everyone for suggestions. I filed a support ticket in parallel. What they and some users on this thread recommended almost completely fixed the issue for me. Here's what I did:
- Login to Evernote Web interface
- Expand my Notebook structure
- Find an alternative notebook, other than my current default, to make the new default, and hit the ... menu and set as default notebook
- Close, and then reopen Evernote app on my Android device
At this point, when I create a new note, it's created in a new default notebook and works just fine. Then, I repeated all the steps above to switch back to my original default notebook. And at the end of it all, I could once again create new notes within my original default notebook.
However, one side effect I noticed after doing all the above, is that in the notebook viewer on my Android device, I now see two entries of my default notebook above/below each other in the list of notebooks. They both have the same note count. And when I access either one it seems to access my default notebook just fine. But it was a very strange side effect of seeing my default notebook listed twice within the list of notebooks. I'm now working with Evernote support to try to fix this new side effect.
Hope all this helps someone else.
-Andy