A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
I spoke with a customer service agent named Ferdie about this issue. After we went through the issue extensively, he informed me it is a server-side issue and has to be fixed on Microsoft's end, and that the details have been brought to their Software Engineers.
Looks like the solution is to sit tight until they fix it on their end.
For now, the "OneNote" app on my windows 10 PC (as opposed to "OneNote for Windows 10") and the onenote accessed via browser (onenote.com) both function normally. That's the band-aid until this gets fixed.