This error is a pretty generic error that indicates somehow the Common Ground instance could not be accessed by the Connector.

The first thing you can try is re-running the sync again. If you continue to see this error, you will need to upgrade your Common Ground or Common Ground Entrerprise.

If this does not work, check Real Time Configuration by going to Data Management -> Import/Export -> Real Time Configuration -> 3. Configure Connection.  If Server
Status reads "Server is not available (null)" do the following:
  1. Log into Common Ground Enterprise.
  2. Go to Setup -> App Setup -> Deploy -> Installed Packages, then click Manage Licenses next to Luminate CRM.
  3. Check if the Convio Connector User has a license assigned.  If not, assign one.  This may require removing a license from another user.