In case your Windows profile user ID contains international characters (for example éöáä and similar), please do the following:
In the server-side:
1. Open Synergy.
2. Click on the File menu and chose "Save the configuration as..."
3. Save the configuration to a directory without the account id, for example, C:\SynergyConfig\Synergy.sgc
4. In the Synergy main window, click on "Use existing configuration."
5. Browse and select the recently created file.
The Synergy application should now be able to read the configuration and start properly.