Cannot authenticate visitors with the Web SDK

I'm trying to use the sample Chat Web SDK app found here https://github.com/zendesk/zopim-chat-web-sdk-sample-app using authentication. I was able to setup the example without authentication but when I use the code found here: https://api.zopim.com/web-sdk/#visitor-authentication in order to authenticate 

zChat.init({
    account_key: 'YOUR_ZENDESK_CHAT_ACCOUNT_KEY',
    authentication: {
        jwt_fn: function(callback) {
            fetch('JWT_TOKEN_ENDPOINT').then(function(res) {
                res.text().then(function(jwt) {
                    callback(jwt);
                });
            });
        }
    }
});

I see the following on the console log:


Zendesk Chat Web SDK: Error: init: Failed to verify token: jwt verification error


The call to: https://id.zopim.com/authenticated/web/jwt returns

{
"success": false,
"error": "failed to validate claims"
}

I know that Im generating the JWT correctly, as I use the same logic for the Zendesk Support SDK and it works. Also, when I validate the token here: https://jwt.io/ it verifies the secret signature and the contents are correct.

The only difference is that for simplicity I'm generating the token inside the jwt_fn instead of calling and endpoint, but that shouldn't be a problem for testing purposes.

Any idea of what I could be missing?

Thanks!!