I've been getting this error on CC Desktop for weeks now and I haven't figured out what is causing it. I have two machines running CC in my home. One is a personal MacBook Pro and the other is a work Lenovo ThinkPad. Both are on the same network but the Lenovo will not sync my libraries. It also doesn't matter if the VPN is disconnected.
My guess is that there is a policy that is blocking some domain from connecting. After looking through log files for adobe domains to check, I found this: http://ns.adobe.com. Does this still do anything in relation to the libraries tool? It is blocked by our computers.