Follow the steps below on the server where the Connector Service is installed.

First, Stop/Restart the Host: 
1. In Scheduler, choose File > Stop Host. 
2. Then, choose File > Start Host. 
3. Try to Initiate Sync Now again.
 
If the stop/start does not work: 
1. Go to Start > Run on the server, then type services.msc. 
2. On this list, double-click "Integration Service #." 
3. Click Stop. 
4. Go back into Scheduler, then check to see if the Sync can be initiated again.