Java network: connection rejected - Yes, my server is running

I get the following error when my client tries to connect to my server socket:

java.net.ConnectException: Connection refused: connect

But my server does work on the same machine. I am trying to connect to it using the external IP address of my router. But when I try to connect with "localhost", it works. And, yes, I correctly passed the port on my router. It canyouseeme.orgcan even connect to my server (the site says “success”, and in my server there is a log that is connected to the server.)

So, for some reason, it is impossible to connect to the same computer (or to a machine on the same network) through an external IP? Or is this something typical of Windows? (I usually use Linux)

I also tried completely disabling the windows firewall.

ServerSocket:

public ServerSocket ssocket;
public List<ClientHandler> handlers;

public Server(int port) { // Constructor
    try {
        ssocket = new ServerSocket(port);
        this.handlers = new ArrayList<ClientHandler>();
        IpSharingManager.uploadData(Utilities.getPublicIp(), port);
    } catch (Exception e) {
        e.printStackTrace();
        System.exit(-1);
    }
}

:

public InvisibleClient(String host, int port) {
    try {
        System.out.println("Trying to connect to " + host + ":" + port);
        this.host = host;
        this.socket = new Socket(host, port);
        this.bis = new BufferedInputStream(this.socket.getInputStream());
        this.bos = new BufferedOutputStream(this.socket.getOutputStream());
        this.console = new RemoteConsole(this.socket);
        initializeCommunication();
        System.out.println("Successfully connected!");
        new Thread(this, "Client Thread").start();
    } catch (Exception e) {
        e.printStackTrace();
        System.out.println("No server available");
    }
}