Split from this thread.
i've the same problem, but didn't get a solution following your steps.
When I try to manually sync, sometimes I've got a code of the error, sometimes I don't.
Now (after doing what you explained) the error code is
Can you help?
Since the problem only happens on OneNote for Mac, we suggest you try the following steps to clear the OneNote credential and check the result.
1. Quit from all Mac Office Applications, except for OneNote.
2. In OneNote, sign out. This only logs you out, and all the notebooks will still be there.
3. Quit OneNote.
4. Launch "Keychain Access" and search for all items that contain "Office15". There should be only a couple items, one for each account you have logged into.
The name will be a long alpha-numeric code and the kind will start with “MicrosoftOffice15…”
6. Delete these items. It is only deleting the authentication credentials, no data. Once you sign in again, they will be re-created.
7. Launch OneNote and sign into your accounts.
Since all the office apps share the credentials (Word/Excel/PowerPoint) you will have to sign in again with those apps as well.