BUG. Impossible to flush the site on live environment.

Calling Session::get() before Session::start() will create an empty current Session object (look at Session::current_session()). And even SilverStripe calls Session::Start() later in /framework/main.php, Session::current_session() will return an empty "cached" Session object. So that's why permission check in /framework/main.php fails and you'll be redirected to login page.

Actually I think that this is bug on SS side but it is faster and easier to make this workaround on fluent side than to do the changes in SilverStripe. :)
BUG Session only starts if necessary