When you use token_auth for authentication on Piwik dashboard the gadgets don't open with error meesage: "Error: You can't access this resource as it requires an 'view' access for the website id = X"
We do a redirection from our site to our Piwik instance specifying good idSite and token_auth:
http://our_piwik_domain/index.php?module=CoreHome&action=index&idSite=X&period=day&date=yesterday&token_auth=some_user_token

The user information is actually loaded but the gadgets don't open.
I've found out a way to fix the problem:
In broadcast.js(line 491):

var ajax = new ajaxHelper();
ajax.setUrl(urlAjax);
ajax._getDefaultPostParams = function () {
    return {};
};

The function _getDefaultPostParams from ajaxHelper.js is overwritten in broadcast.js. When I remove the overwritten version in broadcast.js it works good.