Unity's TCP socket connection attempt "actively refused"

I'm trying to read data from a Java service on an Android device. But Unity always refuses to connect to the TCP socket. It refuses in the editor, it refuses on Android and it refuses if I try to build for Windows. In the Editor, it says: System.Net.Sockets.SocketException: No connection could be made because the target machine actively refused it.

My code is below. It's a adapted from an example from a Microsoft API page. `uPrint` is a function I wrote to print text both in the console and in a GUIText.

The code is called with:

 Connect("127.0.0.1", "Hello!");    

I've tried `"0.0.0.0"` and `"localhost"` for the host name. It didn't work.

I haven't found a way to get at which line exactly the error occurs. Even removing the try/catch blocks does not give me the exact line where it fails.

The Windows firewall allows the Unity editor full access to the network.

     void Connect(string server, string message) {
         try {
             // Create a TcpClient. 
             // Note, for this client to work you need to have a TcpServer  
             // connected to the same address as specified by the server, port 
             // combination.
             int port = 15219;
             TcpClient client = new TcpClient(server, port);
             
             // Translate the passed message into ASCII and store it as a Byte array.
             byte[] data = System.Text.Encoding.ASCII.GetBytes(message);         
             
             // Get a client stream for reading and writing. 
             // Stream stream = client.GetStream(); // GetStream();
             
             NetworkStream stream = client.GetStream();
             
             // Send the message to the connected TcpServer. 
             // stream.Write(data, 0, data.Length);
             
             uPrint(string.Format("Sent: {0}", message));
             
             // Receive the TcpServer.response. 
             
             // Buffer to store the response bytes.
             data = new Byte[256];
             
             // String to store the response ASCII representation.
             string responseData = string.Empty;
             
             // Read the first batch of the TcpServer response bytes.
             int bytes = stream.Read(data, 0, data.Length);
             responseData = System.Text.Encoding.ASCII.GetString(data, 0, bytes);
             uPrint(string.Format("Received: {0}", responseData));
             
             // Close everything.
             stream.Close();         
             client.Close();         
         } 
         catch (ArgumentNullException e) {
             uPrint(string.Format("ArgumentNullException: {0}", e));
         }
         catch (SocketException e) {
             uPrint(string.Format("SocketException: {0}", e));
         }        
     }