I fixed this by using the following process:

Made multiple copies of my OneNote file.

Decided to try deleting a few sections at a time and exporting. After deleting the first three sections, I was able to export successfully.

Opened another copy of my OneNote file and deleted each of the three suspect sections, one at a time. Narrowed it down to a single section. Even after deleting all text and pages (but leaving the section in place), I still couldn't export.
So, I deleted that section and could successfully export.

I then used another copy to rebuild that one section and could still export at that point. Something definitely got corrupted with that one section.