A Microsoft file hosting and synchronization service.
Thanks! I'd been doing the same workaround when OneDrive stopped working with old Mac OSs, but had to reinstall weekly due to the pesky standardaloneupdater rearing it's ugly head.
I've now made the file edit in terminal to
kill the autouodates, as per https://answers.microsoft.com/en-us/msoffice/forum/all/my-fix-for-onedrive-on-macos-catalina-1015x/de6ab37d-fa23-42b5-b5df-fbf05e09887b and problem seems to be solved for the moment. Turning off auto updates in OneNote preferences has also been suggested, and I'll do that too.
What a ridiculous situation.