Official comment
Diego,
When you exclude a folder it stops syncing with the Syncplicity servers, but leaves the existing contents on the local system. To start syncing it again you can click on the Syncplicity icon in the system tray, select Manage, and navigate to the folder in the Manage window that appears. Click on it, and the options on the right side of the screen will include "Sync to this computer" which will start the syncing again if you click on that button.
There is more information and screenshots of this in this article:
https://syncplicity.zendesk.com/hc/en-us/articles/201239080-Working-with-folders-and-files