plugins/RssWidget/RssRenderer.php indicates that the error occurs in the widget that fetches the piwik rss feed to display news inside of Piwik.
Warning - simplexml_load_string(): means that the piwik website retured something that couldn’t be parsed as a RSS feed.
As multiple people had the same issue at the same time, I guess the Piwik website was offline or had an error for a short time and your Piwik instances tried to update the feed in this moment.
I created a github issue about not throwing an error in this case: