Since yesterday I can't save e-mails directly from Outlook to OneNote anymore, I get the following error message: "Error when expanding notebook. At least one document library in the user's or group's OneDrive contains more than 5,000 OneNote items (notebooks, sections, section groups) and cannot be queried using the API. Make sure that none of the user's or group's document libraries contains more than 5,000 OneNote items. Go to this page for more information: https://blogs.msdn.microsoft.com/onenotedev/2016/09/11/onenote-api-calls-fail-with-a-large-number-of...". I may have 5,000 notes, but definitely no 5,000 sections, section groups or even notebooks!
I also get a similar error message in the OneNote web clipper: "We couldn't load your notebooks because a list limit on OneDrive was exceeded."
Astonishingly, working directly in OneNote works perfectly normally - both in the OneNote app and in OneNote 2016.