Content similar to the following is being added to an article :

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
</head>
<body>
</body>
</html>

This is most likely caused by the Fullpage plugin which you have recently installed. The Fullpage plugin allows you to edit all the elements of an HTML page. This is useful in some cases, such as when creating an e-mail in a Newsletter extension but is not required for a normal Joomla! article.

Uninstall the Fullpage plugin using the JCE Installer or move the Fullpage button out of the Editor Layout.