I have been getting that error sporatically as well. When it happens, a combination of resetting Office from Settings/Applications/Office and also launching the Skydrive app and navigating to the Outlook file in Documents seems to eventually reconnect the sync. No idea what the problem is, but it's happened 3 times over the last month or so, and the "fix" works for a while.
I also have OneNote syncing to my iPad and that's been solid as a rock through this.
I've been having issues getting one note to sync for ages now, I even went to the extreme and fully reset my phone but still no joy. On first use onenote tries to upload the default notebook to skydrive but fails.
It also does not find my existing notebooks on skydrive, so mine isn't just a 'sync' issue it simply wont allow me to set it up.
One note works perfectly fine via desktop app (office 2013 beta) and via windows 8 touch app.
The annoying thing is the apps for iPhone and android work fine, but due to onenote being bundled with office on WP7.5 it gets no updates, so I assume MS are aware but cant do anything, frustrating.