Here I am again :)
I discovered a few new issues. The plugin a CSS file twice: "wp-rss-aggregator/css/colorbox.css". Only once would be fine... but more than that, I wouldn't load it on every page on the website, but only on the page where I am using the plugin (the css could be loaded when the plugin is called, doesn't absolutely need to be in the header in this case). Moreover, I personally don't need that CSS at all, so maybe you could had a checkbox to disable it in the settings?
Linked to that, there are two 404 errors with those images: 'wp-rss-aggregator/css/images/border.png' and 'wp-rss-aggregator/css/images/controls.png'.
That would be two great things to improve for your next update ^^
Thanks a lot,