Socket.IO application not starting, no output

I have the following simple socket.io app :

var fs = require('fs');
var db = require("./libs/db.js");

var sslOptions = {
    key: fs.readFileSync('/var/ssl/server.key'),
    cert: fs.readFileSync('/var/ssl/server.crt'),
    ca: fs.readFileSync('/var/ssl/ca.crt'),
    requestCert: true,
    rejectUnauthorized: false
};

var io = require('socket.io').listen(4000,sslOptions);

It runs, displaying : info: socket.io started on launch. The page example.com:4000 outputs : Welcome to socket.io, all good.

Now, in another directory, I have :

var fs = require('fs');

var sslOptions = {
    key: fs.readFileSync('/var/ssl/server.key'),
    cert: fs.readFileSync('/var/ssl/server.crt'),
    ca: fs.readFileSync('/var/ssl/ca.crt'),
    requestCert: true,
    rejectUnauthorized: false
};

var io = require('socket.io').listen(2000,sslOptions);

That code (exactly the same??) doesn't output anything on launch...

socket.io versions are identical (1.1.0), both directories are chmoded the same. What could cause such behaviour? How can I debug?

Edit :

When I cp the not working app in the same dir than the working app, it runs... Again, permissions are the same.