Can't set a signed SSL certificate on NodeJS/expressJS

I'm trying to set a signed certificate to set an HTTPS sever on nodejs. But I keep getting the following error in chrome (and similar on other browsers).

"Error 107 (net::ERR_SSL_PROTOCOL_ERROR): SSL protocol error."

When I ran the same code with self signed certificate and it worked OK.

Then I ran openssl verify and seems like the certificate is OK.

$ openssl verify -CAfile ./ca.crt server.crt
server.crt: OK

This is how I run it with the signed certificate:

    var privateKey  = fs.readFileSync('sslcert/server.key').toString();
    var certificate = fs.readFileSync('sslcert/server.crt').toString();
    var ca = fs.readFileSync('sslcert/ca.crt').toString();
    var credentials = {key: privateKey, cert: certificate, ca: ca};

    //Start Server
    https.createServer(credentials, app).listen(httpsPort, function () {
        console.log("Listening on port " + httpsPort + ' (https)');
    });