Hi Martin,
Thanks for your post in Microsoft Community.
Not sure which method did you use to embed the OneNote file to the SharePoint page.
But you may try the method I use. It works like a charm.
Go to the SharePoint/OneDrive for Business and find the OneNote notebook which you would like to embed. Click on Share > Anyone permission. Open the Incognito mode of your browser and access the sharing URL. After the notebook is fully loaded. Click on the URL bar and copy the URL.
Now let's delete the unnecessary part of the URL and the final result should like the format as below:
https://Contoso.sharepoint.com/sites/SiteName/_layouts/15/Doc.aspx?sourcedoc={x xxx-xxxx-xxxx-xxxx-xxxx}
Add it to the iframe embed code
<iframe height="500" width="700" src="https://Contoso.sharepoint.com/sites/SiteName/_layouts/15/Doc.aspx?sourcedoc={xxxx-xxxx-xxxx-xxxx-xxxx}></iframe>
Go to SharePoint page, insert the embed web part and paste the iframe code into it. It should load the notebook without issue.
Regards,
Alex Chen
Regards,
Alex Chen