I continued spending time on this, looking for a solution. One of the posts for the Windows version suggested logging out and logging in. I couldn't log out (infinite loop), but at the upper right corner of the OneNote window on Mac you will see a message about sync errors you can click. Interestingly, it said a section was "too big." I created new sections and moved material to them from the "too big" section and it seems to be synchronizing now (has been going 1/2 hour and has uploaded 2.1GB so far.
So... if there are section size limits, why are they not well known?
If there are section size limits, why does OneNote not flag them when you hit them versus requiring experimenting and debugging to find them?
Most importantly, why would there need to be section size limits in the first place?
Thanks,
Greg Kovacs
Stanford University