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Hello Don Le
Since the issue affects most of your notebooks and your OneDrive storage is not full, this may be broader than a single problematic section name.
Could you please confirm the following?
- Do the affected notebooks open normally in OneNote for the web, and do they contain your latest changes?
- Do those same notebooks sync correctly on another device, such as a phone or another computer?
- Is the error limited to the OneNote desktop app on one Windows computer?
- Are the notebooks stored in a personal OneDrive account, a work or school account, or SharePoint?
- Which OneNote version and build are you using? You can find this under File > Account > About OneNote.
- If you create a small test notebook in OneNote for the web, does it appear and sync in the desktop app?
These results will help narrow down the cause:
- If the notebooks work on the web and other devices, the issue is likely isolated to the desktop app or its connection on that computer.
- If they also fail on the web or other devices, the issue may be related to the account, notebook storage location, permissions, or the Microsoft service.
Meanwhile, install any available Office updates from File > Account > Update Options > Update Now. Please do not sign out, reset OneNote, clear its cache, or uninstall it until you have confirmed that all recent notes are safely available online or backed up.
If the notebooks fail across the web and multiple devices, this may require Microsoft Support to inspect the account-side synchronization problem, since Microsoft Q&A contributors cannot access OneNote service logs.
Please reply with the results of the web and second-device tests, together with your OneNote version, and we can narrow down the next step.
I hope to hear update from you soon.