This is what fixed the problem for me:
This link describes it quite well:
https://support.office.com/en-us/article/fix-issues-when-you-can-t-sync-onenote-299495ef-66d1-448f-90c1-b785a6968d45
What really matters, if your onenote (windows 10 app) has syncing problems:
D o you have important information in your notes (on your local device) that hasn't synced with your Onedrive/ sharepoint versions?
Checkt this by accessing the Onedrive/ sharepoint versions via oneDrive web-page or by right clicking on the notebook and choose "copy link to notebook" and paste the link to browser.
If yes:
Don't close your notebooks in the app or log off with your windows account, as this leads to the loss of your current version of notes( that are locally saved in your cache and cache gets deleted).
Instead open the Onedrive/ sharepoint versions (accessible via web-page) in the app of your device.
If possible move sections/ pages of the notebook that will not sync to the notebook opened via Onedrive/ sharepoint (this one should sync fine). If you transferred all important information to your newly opened notebook, delete the old one that doesn't sync.
Now you should have all the original info that wouldn't sync in a notebook that does sync.
If no:
You have nothing to fear. Try to close the notebook and reopen it again. Or try to log off with your windows account and log in again.
If you don't use the windows 10 app and use a version that still allows local saving of notebooks (like onenote 2016):
Save your Notebook that has problem syncing with the cloud locally on your device. If this works you can share or move your notebook from there.
This hopefully helps.