2 weeks ago
Interesting. I have not updated to High Sierra (Still on 10.12.6) and out of curiosity checked my endnote 8 sync. SURPRISE! I am also not syncing from desktop (MAC) to web. My tablet and browser are on web and are missing all references I have added to desktop since late August. Just spent an hour trying to resolve that with sync settings on desktop to no avail.
Is it possible you also were not syncing prior to your upgrade to 10.13 OS?