We have noticed this issue could be caused by the Secure Input feature on your mac.
macOS allows apps to turn on Secure Input when the user needs to type any sensitive information (eg. a password). This is a security feature which helps to prevent any malicious app from getting access to such information.
When an app turns it on, only that app can read the user's keystrokes. Synergy is also unable to read keyboard input, and therefore send the keystrokes to the client.
Most of the time, this is fine because as soon as you have finished entering your password or other sensitive information, the app will turn off Secure Input and Synergy will start sending the keyboard input to your client again.
However, some apps have bugs and they do not turn off Secure Input in some cases.
When this happens, quitting and relaunching the app will turn off Secure Input and allow Synergy to be able to read and send keystrokes again.
You may also run the following command to see which application is using Secure Input and close it: ioreg -l -w 0 | grep SecureInput
If this article didn't help, you can talk to someone on our support team (unless we sent you here, in which case please reply via the ticket you already have open).