A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
(I'm a.k.a. iggleXX. Can't recall exactly why I had to create a new account to post yesterday.)
Tisky, before your reply I did close the offending notebook, and got an error that it hadn't synced and the problem section was put at the end of the notebooks as "Misplaced sections" which could be opened read-only.
Then after various experiments, such as copying whole pages to a new notebook, that failed with the same issue, I was able to solve the problem by tediously creating new pages in a new notebook and copying/pasting the contents of the old pages.
If the problem recurs I'll try your suggestions.