Ошибка на post запрос. React express приложение

С клиента, написанного на react, отправляю post запрос

const addNewItem = async (e) => {
        try {
            const response = await fetch('/users', {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json'
                },
                body: JSON.stringify(newElementFields),
            })
            const result = await response.json();
            console.log(result);
        } catch (e) {
            throw new Error();
        }
    }

на express сервер:

const app = express();
const port = process.env.PORT || 4320;

app.use(express.static(path.join(__dirname, '..', 'dist')));
app.use(express.json());

app.get('/users', async (req, res) => {
    try {
        const users = await readFileAsync('data', 'users.json');
        res.status(200).send(users);    
    } catch (e) {
        res.status(500).json({
            message: 'Server error',
        });
    }
})

app.post('/users', async (req, res) => {
    try {
        await writeFileAsync('data', 'users.json', req.body);

        res.status(200);
    } catch (e) {
        res.status(500).json({
            message: 'Server error'
        })
    }
    
})

В итоге в браузере получаю ошибку 504, а на сервере:

Error occurred while trying to proxy request /users from localhost:4120 to http://localhost:4320/ (ECONNRESET) (https://nodejs.org/api/errors.html#errors_common_system_errors) 

Подскажите пожалуйста, как можно это решить?

Webpack dev-server вроде настроен:

 devServer: {
    historyApiFallback: true,
    port: 4120,
    open: isWindows ? 'chrome' : 'google-chrome',
    proxy: {
      '/users': 'http://localhost:4320/',
      "secure": false,
      "changeOrigin": true
    },
  },

Package.json тоже:

"scripts": {
    "server": "nodemon server/index.js",
    "devFront": "cross-env NODE_ENV=development webpack --mode development",
    "dev-server": "cross-env NODE_ENV=development webpack-dev-server --mode development --open",
    "dev": "concurrently --kill-others-on-fail \"npm run server\" \"npm run dev-server\"",
    "heroku-postbuild": "NPM_CONFIG_PRODUCTION=false webpack -p"
  },
  "proxy": {
    "/users": "http://localhost:4320/",
    "secure": false,
    "changeOrigin": true
  },