Input del form nulli

Ciao a tutti, io ho riscontrato un problema cercando di fare il login: quando invio il form al server, la servlet che controlla le credenziali (CheckLogin) mi restituisce l'errore secondo il quale le credenziali inserite sono nulle, e non capisco perchè. Ho provato a debuggare da Chrome e riesco a vedere i valori del form.

La servlet CheckLogin:

package it.tiw.controllers;

import java.io.IOException;
import java.sql.Connection;
import java.sql.SQLException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.lang.StringEscapeUtils;

import it.tiw.beans.User;
import it.tiw.dao.UserDAO;
import it.tiw.utils.ConnectionHandler;

@WebServlet("/CheckLogin")
public class CheckLogin extends HttpServlet {
	private static final long serialVersionUID = 1L;
	private Connection connection = null;

    public CheckLogin() {
        super();
    }

    public void init() throws ServletException {
    	connection = ConnectionHandler.getConnection(getServletContext());
    }
    
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		String username = null;
		String password = null;
		username = StringEscapeUtils.escapeJava(request.getParameter("username"));
		password = StringEscapeUtils.escapeJava(request.getParameter("pwd"));
		if(username == null || password == null || username.isEmpty() || password.isEmpty()) {
			response.setStatus(HttpServletResponse.SC_BAD_REQUEST);
			response.getWriter().println("Credentials cannot be null");
			return;
		}
		
		UserDAO userDao = new UserDAO(connection);
		User user = null;
		try {
			user = userDao.checkCredentials(username, password);
		} catch(SQLException e) {
			response.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
			response.getWriter().println("Not possible to check credentials at the moment, try later");
			return;
		}
		
		if (user == null) {
			response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
			response.getWriter().println("Incorrect credentials");
		}
		else {
			request.getSession().setAttribute("user", user);
			response.setStatus(HttpServletResponse.SC_OK);
			response.setContentType("application/json");
			response.setCharacterEncoding("UTF-8");
			response.getWriter().println(username);
		}
	}
	
	public void destroy() {
		try {
			ConnectionHandler.closeConnection(connection);
		} catch(SQLException e) {
			e.printStackTrace();
		}
	}

}

script login (loginManag.js):

(function() {

    document.getElementById("loginButton").addEventListener('click', (e) => {
        var form = new FormData(e.target.closest('form'));
        if(form.checkValidity()) {
            makeCall("POST", 'CheckLogin', e.target.closest('form'), 
                function(req) {
                    if(req.readyState == XMLHttpRequest.DONE) {
                        var message = req.responseText;
                        switch(req.status) {
                            case 200: //ok request
                                sessionStorage.setItem("username", message);
                                window.location.href = "Home.html";
                                break;
                            case 400: //bad request
                                document.getElementById("errorMessageLogin").textContent = message;
                                break;
                            case 401: //unauthorized
                                document.getElementById("errorMessageLogin").textContent = message;
                                break;
                            case 500: //server error
                                document.getElementById("errorMessageLogin").textContent = message;
                                break;
                        }
                    }
                });
        } else {
            form.reportValidity();
        }
    });
})();

script che fa chiamate al server (utils.js):

	function makeCall(method, url, formElement, cback, reset = true) {
	    var req = new XMLHttpRequest(); // visible by closure
	    req.onreadystatechange = function() {
	      cback(req)
	    }; // closure
	    req.open(method, url);
	    if (formElement == null) {
	      req.send();
	    } else {
	      req.send(new FormData(formElement));
	    }
	    if (formElement !== null && reset === true) {
	      formElement.reset();
	    }
	  }

file html con form:

<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Login Page</title>
<script src="utils.js" charset="utf-8" defer></script>
<script src="loginManag.js" charset="utf-8" defer></script>
</head>
<body>
<h1>Welcome to the Money Transfer Application!!!!</h1>
	
	<form action="#">
		<fieldset>
			USERNAME: <input type="text" name="username" required> <br>
			PASSWORD: <input type="password" name="pwd" required><br>			
			<input id="loginButton" type="button" value="LOGIN">
			<p id="errorMessageLogin"></p>
		</fieldset>
	</form>
</body>
</html>