Kept on getting an "access denied" error while loading a page in 15 hive ( _Layouts" folder.) I was using "window.load" javascript function to open this page in full page mode.  The solution was to refer to this page with the full url aka  http://SiteCollectionUrl/_layouts/pageName.aspx and not a relative url like "_layouts/pageName.aspx"

Don't know exactly the reason but my guess is that it has something to do with cross site scripting. Anyways hope this helps you

cheers