How can i connect to node server with letsencrypt https activated

Question

I have a problem since i activated letsencrypt on my domain and did'nt have problem with http server before.

Here is my app.js code:

var app = require('express')();
var fs = require('fs');
var https = require('https');

var secureServer = https.createServer({
    key: fs.readFileSync('server.key'),
    cert: fs.readFileSync('server.cert'),
    ca: fs.readFileSync('server.cacert'),
    requestCert: true,
    rejectUnauthorized: false
}, app).listen(5221, function() {
    console.log("Secure Express server listening on port "+ 5221);
});
var io = require('socket.io')(secureServer);

The Secure Express server listening on port 5221 prints out but nothing more and the codes in:

io.on('connection', function (socket) {
    console.log(`Socket ${socket.id} connected.`);
}

Is not working at all.

I've also tested with .pem files, with ca.crt or without that... but nothing changes.