If you do a forum search you’ll find that others have had this issue of WD Sync not properly working with multiple computers syncing to the same location on the My Cloud. The solution appears to be that one computer has to finish syncing BEFORE the next computer starts its sync otherwise the sync database stored on the My Cloud becomes corrupted which causes the sync to fail. The WD Sync software currently really isn’t designed to sync multiple computers from one single location on the My Cloud, rather its designed for each computer to sync to their own separate sync folder.
More than a few have dumped the WD Sync software (for a variety of reasons) and have gone with 3rd party software like Free File Sync (http://www.freefilesync.org/).
Here are a few past threads on the issue some have had success others have not when using WD Sync.
https://community.wd.com/t/wd-sync-not-syncing-between-2-computers/141218
https://community.wd.com/t/sync-between-folders-on-different-pcs/149985
https://community.wd.com/t/how-to-sync-two-mycloud-at-different-locations/145528
https://community.wd.com/t/syncing-folders-on-wd-mycloud/95486
