Hi @frob ,
Thanks for your explanation.
Did you or some users sync a document library on SharePoint to OneDrive client?
Here is one way to set the default location for notebooks, open OneNote > Go to File > Options > Save & Backup, then modify the "Default Notebook Location", choose one folder from SharePoint, which is like the folder of "Team1 - Doc127" on following image.
Hope the information is helpful to you.
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