I encountered this problem when I'd deleted my entire library on one machine then tried to sync it. I presume you've done something similar?

Clicking on the gear button in the toolbar, then selecting "Clear Server Data" solved it for me. Make sure, before you do that, that you have a full copy of your library on one machine. Sync that machine again, then sync the other.

That worked for me, anyway. I'm not sure exactly what led to your circumstance. More information is always helpful.