File header.tpl in admin/view/template/common has the following line commented out:
<!--
<link href="view/javascript/bootstrap/css/bootstrap.css" rel="stylesheet" media="screen" />
//-->
Remove <!-- and //--> and all the problems go away.
method 2.
Fixed. I had to add the MIME type .less text/less to the Plesk control panel. To do this, navigate to Virtual Directory, then the Mime Types tab. Click add mime type then add the type.
