I found the root cause by looking into the "Microsoft Entra" portal for my M365 tenant.
In "Enterprise Application" there's a sign-in log available.
It shows that since 2023-10-01 One Note Web Clipper has been failing
and it has been failing because someone at Microsoft needs to update the "secret keys" for the OneNote Web Clipper application(s)
So there problem is nothing at all to do with anything on peoples local machines, or their browser settings etc. etc.
Explains why several many people suddenly get the error at the same time (more or less)
No idea though how to get this information through to the right people at Microsoft
- one they know it will be a 1 minute fix - just update the relevant client secret keys
