How to fix net::ERR_EMPTY_RESPONSE for POST request on React/Node app

Question

I tried other solutions but still no luck. I'm adding a simple contact form on my React/Node app using Nodemailer. I'm using Heroku to deploy the app and currently testing on my local server. Although a message is delivered to the assigned mailbox, I keep getting errors net::ERR_EMPTY_RESPONSE and TypeError: Failed to fetch after the Node.js core's 2 minute timeout.

I have tried 1) both axios and fetch request on the client side and 2) adding CORS headers in the server file but the same results.

app.use(function(req, res, next) {
    console.log('request', req.url, req.body, req.method);
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Request-Method: GET,PUT,POST,DELETE,OPTIONS")
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept, x-token");
    if(req.method === 'OPTIONS') {
        res.end();
    } else {
        next();
    }
});

This is the submit handler method from the client side:

handleFormSubmit(e){
        e.preventDefault();
        console.log(this.state);
        console.log('submit clicked');
        const data = {
            "fname": this.state.fname,
            "lname": this.state.lname,
            "email": this.state.email,
            "message": this.state.message
        }

        fetch("http://localhost:3000/api/form", {
            method: "POST",
            headers: {"Content-Type": "application/json",
                      "Access-Control-Allow-Origin": "*"
                     },
            body: JSON.stringify(data)
        })
        .then((res) => {return res.json()})
        .then(res => {
            if(res.ok){
                console.log("request sucess");
                return res.json();
            } else {
                console.log("request fail");
                throw new Error("something went wrong..");
            }
        }).catch(err => {
            console.log(err);
            console.log("error happed");
        })

I followed the Nodemailer tutorial and the routing is set correctly. The message arrives on the mailbox. I expect the output to return success but keep catching the error and I don't know what I'm doing wrong. I really appreciate for any advice.