Policy File is being served, but never recognized

I am trying to serve a crossdomain policy file from a Java server to the Unity webplayer. When the webplayer connects to the server, the console (server-side) shows: "ServingPolicyFile CONNECTION MADE", but the webplayer throws an exception:

 SecurityException: Unable to connect, as no valid crossdomain policy was found
 System.Net.Sockets.Socket.Connect_internal (IntPtr sock, System.Net.SocketAddress sa, System.Int32& error, Boolean requireSocketPolicyFile)
 System.Net.Sockets.Socket.Connect_internal (IntPtr sock, System.Net.SocketAddress sa, System.Int32& error)
 System.Net.Sockets.Socket.Connect (System.Net.IPAddress[] addresses, Int32 port)
 System.Net.Sockets.Socket.Connect (System.String host, Int32 port)

Here is the server code (Java)

 public class ServingPolicyFile {
 
     final private static int PORT = 843;
     final private static String POLICY_FILE_FORMAT = "<?xml version='1.0'?><cross-domain-policy><allow-access-from domain=\"%s\" to-ports=\"%s\"/></cross-domain-policy>\0";
     final private ExecutorService _executor = Executors.newSingleThreadExecutor();
     final private ServerSocket _servingSocket;
     final private String _policyFile;
 
     public ServingPolicyFile(String policyFile) throws IOException {
         _servingSocket = new ServerSocket(PORT);
         _policyFile = policyFile;
     }
 
     public ServingPolicyFile(int port) throws IOException {
         this(String.format(POLICY_FILE_FORMAT, "*", port));
     }
 
     public void close() throws IOException {
         _executor.shutdownNow();
         _servingSocket.close();
     }
 
     public void start() {
         _executor.submit(() -> {
             while (true) {
                 try (Socket socket = _servingSocket.accept(); PrintWriter out = new PrintWriter(socket.getOutputStream(), true)) {
                     System.out.println("ServingPolicyFile CONNECTION MADE");
                     out.println(_policyFile);
                 }
             }
         });
     }
 }
 

Does anyone know what the issue might be?

I tried serving the EXACT policy file (copy & pasted) from the unity docs, but that didn't work either.