I had the same issue and it was because "Zimbra Desktop Service" was not started on my computer.
For Windows, right-click on "My Computer" and select "Manage". You should then see "Services and Applications" in the tree view on the left. Expand the tree and click on "Services", then look for the "Zimbra Desktop Service".
If it does not list it as "Started", then right-click the service and choose "Start".
Hope this helps...
-knitcap |