I have a mycloud mirror. My daughter, who does not live at home, uses WD sync to sync files from her laptop to the mycloud. Every thing worked ok for about 18 months but since +/- halfway november 2016 the sync no longer works.
On her laptop I see the message unable to connect extern storage.
Make sure that Cloud Access on the Cloud is still setup properly.
Check the router settings where she is to see if anything has changed; things like upnp, file sharing(on the PC), firewalls, ports for remote access are open, etc…
Also try to uninstall and reinstall WD Sync.