OneNote Stopped syncing. We can't sync this section because the server isn't compatible with this version of OneNote.

While attempting to further diagnose this issue, I had a small breakthrough.

While attempting different things to night, my existing notebooks on OneDrive began synching after I re-enabled Files On-Demand (further context for it being off is provided below).  Although my two OneDrive notebooks are now synching again, my existing shared notebook that is an Office 365 Group SharePoint location still will not synch.

I haven't had time to prove whether this was coincidental or a true root cause, so I'll be very interested to hear whether this makes any difference for the other users experiencing this issue.

Additional OneDrive context... around the end of February or early March I ultimately had to turn off OneDrive Files On-Demand.  My OneDrive client, which also happens to be an insider version, was constantly hogging my CPU and, more importantly, destroying my battery life.  I tried several potential resolutions from the forums, including a complete reset and re-install of OneDrive, but there was either no change or the behavior returned after 12-24 hours.  Turning off Files On-Demand solved this issues.  Prior to tonight, that feature has been off since.

To my surprise, all existing notebooks, including the SharePoint notebook, are now synching this morning.

It is possible the OneDrive Files On-Demand setting change is more than coincidence.

My thinking, and others please correct me if I'm wrong, has always been that syncing OneNote notebooks via the OneDrive sync client is a bad idea. My thinking is that a local cache is created by the client, so pulling down a copy would likely just see constant updates as the file is worked, and it would sync basically endlessly. With the client creating a local copy, it seems unnecessary.

For those reasons, I've always explicitly excluded my Notebooks directory from syncing locally via the OneDrive client. That being the case, Files-on-demand is out-of-play for me.

BR