I found out why.
For OneDrive/OneNote business env, OneNote business contents are synched into OneDrive storage, on the 'Notebooks' folder.
The customer had some local OneNote files, created by locally working OneNote S/W. And moved the whole Notebook folder to the above 'Notebooks' folder of OneDrive business storage.
That was the problem. After I moved that manual/local folder out of the 'Notebooks' folder, the error was gone.
Now I escaped this situation, but it worries me when I think about this way of OneNote/OneDrive business synch, and Graph API.
With OneDrive API I can download the whole contents of the 'Notebooks' folder. Actually it's a Folder with some OneNote related *.TOC file and *.ONE file. But from the OneDrive web page, it's shown as a 'shortcut' to the OneNote page.
And with OneDrive API, you can not restore that content that you have just downloaded by the GET API. If you intentionally create a folder and upload the *.TOC and *.ONE content by using Graph API, the whole OneNote site goes malfunction.
I have written an opinion on this limitation (or problem) to the suggestion page, asked a question on Git-hub, and even met MS engineers and told this issue, but for years the situation is the same.
Seriously, this will become a big problem.