Error code 11: this error code typically means that Synergy permissions to accessibility are not valid and need to be enabled again (possibly due to an OS update).
On your Mac:
- Stop Synergy and Quit
- Open MacOS System Preferences > Security and Privacy > Accessibility
- Click on the Lock icon on the bottom left of the window and enter your username and password
- Locate Synergy in the list, uncheck it and recheck it
- Open and Start Synergy