XML Parsing Error when trying to log in via Skype for Business Web SDK

I'm trying to add skype for business Web SDK to a website, and I copied and modified the (erroneous) log-in snippet from here

When I run the code in Firefox I get this error:

My code looks like this:

File: Home.html, header section

<!-- skype SDK -->
<script src="https://swx.cdn.skype.com/shared/v/1.2.15/SkypeBootstrap.min.js"></script>
<script src="../../Scripts/SkypeForBusiness.js"></script><!-- my skype log-in code -->

File: SkypeForBusiness.js

$(document).ready(function () {
    console.log("initializing skype...");
    Skype.initialize({ apiKey: config.apiKey }, function (api) {
        window.skypeWebApp = new api.application();
        //Make sign in table appear

        window.skypeWebApp.signInManager.signIn({
            username: 'my.username@mycompany.com',
            password: 'myPassword'
        }).then(() => {
            console.log('Signed in as ' +
                window.skypeWebApp.personsAndGroupsManager.mePerson.name());
        });

        // whenever client.state changes, display its value
        window.skypeWebApp.signInManager.state.changed(function (state) {
            console.log(state);
        });

    }, function (err) {
        console.log(err);
        alert('Cannot load the SDK.');
    });
});

I also looked in the Network view of the Firefox Debugger window and saw this POST call to https://browser.pipe.aria.microsoft.com/Collector/3.0/: which responds with 200 ok status but with no content. One of the parameters is also "client-id: 'NO_AUTH'" and I wonder if this is right.

I have installed the Skype for Business Client on the same computer and successfully logged in with the same credentials I use in my code.

I have two Questions:

1. What is the root of the error and how to solve it?
2. It would be wonderful if someone could provide some links with tutorials and or good examples for this SDK.

thanks in advance! :)