The problem could be described with the following behavior:
·         Cannot insert a link with the Link Manager;
·         If switched to HTML mode cannot return to Design mode, switching to Preview – allows Design mode.
·         A JavaScript error is thrown on focusing and on editing the content area. 

Possible fixes are: 
o   Removing the Modernizr JavaScript file;
o   Setting the RadEditor with ContentAreaMode="Iframe" property;