"You failed to send a correct heartbeat"
means the files you are using are either outdated or edited.
Basically, it checks the files of the server and the files in your Freestyle GunZ folder, if they don't match, you'll get an error.
Or your anti-virus / firewall is blocking the client to send the correct information.
But since you're saying you don't have any anti-virus program, did you turn your firewall off?