There's an error in the configuration file. This error is always accompanied by another message describing the problem. Use that message and the configuration documentation to determine the fix.
The client was able to contact the server computer but the server was not listening for clients. Possible reasons are:
Make sure the client is using the hostname or IP address of the computer running the synergy server.
Make sure the synergy server is running on the server computer. Make sure the server is ready to accept connections. If another program is using synergy's port (24800 by default) then synergy can't start unless you specify a different port.
Synergy uses port 24800 by default but you can specify a different port. If you do use a different port you must use that port on the server and all clients.
The most likely reasons for this are:
A firewall is a program or device that deliberately blocks network connections for security reasons. Typically, they'll silently drop packets they don't want rather than sending a rejection to the sender. This makes it more difficult for intruders to break in.
When synergy traffic hits a firewall and gets dropped, eventually the synergy client will give up waiting for a response and time out. To allow synergy traffic through first find all the firewalls on the network between and on the synergy client and server computers.
A firewall on the server or any network device between the server and any client should allow packets to TCP port 24800. (Port 24800 is the default; use whichever port you've selected.) You'll have to consult the manual for your operating system, device, or firewall software to find out how to do this.
Usually you'll won't need to adjust a firewall on client machines. That's because firewalls normally allow incoming traffic on any port they've initiated a connection on. The reasoning is, of course, if you started a conversation you probably want to hear the reply.
Correct the network problem and try again. You might try ping to see if the two computers can see each other on the network.
If the synergy server is running but locked up or very busy then the client may get this message. If the server is locked up then you'll probably have to restart it. If it's just very busy then the client should successfully connect automatically once the server settles down.
Synergy tried to start listening for clients but the network port is unavailable for some reason. Typical reasons are:
You must have a TCP/IP network device installed and enabled to use synergy.
Check that a synergy server isn't already running.
Only one program at a time can listen for connections on a given port. If the specific error is that the address is already in use and you've ruled out the other causes, then it's likely another program is already using synergy's port. By default synergy uses port 24800. Try having synergy use a different port number, like 24801 or 24900. Note that the server and all clients must use the same port number. Alternatively, find the other program and stop it or have it use another port.
This error can be reported when reading the configuration; see cannot read configuration. If the configuration was read successfully and you get this error then it means that the server's screen is not in the configuration. All screens must be listed in the configuration.
A common reason for this is when you haven't used the system's hostname as its screen name. By default, synergy uses the hostname as the screen name. If you used a different screen name in the configuration then you must tell synergy what that name is. Let's say the hostname is frederick but the configuration defines a screen named fred. Then you must tell the server that its screen name is fred by using the --name fred command line option or setting the screen name in the advanced options dialog to fred.
Alternatively, you can specify one name as an alias of another. See the configuration documentation for details.
Another common reason for this is a mismatch between what you think the hostname is and what synergy thinks it is. Typically this is a problem with fully qualified domain names (FQDN). Perhaps you think your system is named fred but synergy thinks it's fred.nowhere.com or fred.local. You can use either solution above to fix this.
The client is using a screen name not in the server's configuration. This is essentially the same problem as Unknown screen name "XXX" and has the same solutions: specify another screen name or add an alias.
This happens when:
Each client must have a unique screen name. Configure at least one client to use a different screen name.
It's possible for a client to disconnect without the server knowing, usually by being disconnected from the network or possibly by going to sleep or even crashing. The server is left thinking the client is still connected so when the client reconnects the server will think this is a different client using the same name. Synergy will usually detect and correct this problem within a few seconds. If it doesn't then restart the server.
You're using different versions of synergy on the client and server. You should use the same version on all systems.
This is FAQ #17 and is also mentioned in the documentation for using synergy and configuration.