Twitter has changed the URL to access your RSS feed. The plug-in is using an old URL that has been deprecated.
In your installation, go to wp-content/plugins/shortcodes-ultimate/lib/twitter.php
Find the line:
$messages = fetch_rss( 'https://twitter.com/statuses/user_timeline/' . $username . '.rss' );
And change it by:
$messages = fetch_rss( 'https://api.twitter.com/1/statuses/user_timeline.rss?screen_name=' . $username);
