Login Box - Give a Blank Screen

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I have just in stalled the LOGIN BOX block from the Marketplace. When I go to insert it into my page it shows up fine. When I save edits the page goes blank and nothing shows up.

My page source looks like this:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8" />
<!-- insert CSS for Default Concrete Theme //-->
<script type="text/javascript" src="/concrete/js/jquery.js"></script>
<style type="text/css">@import "/concrete/css/ccm.default.theme.css";</style>
</head>
<body> 
<div id="ccm-theme-wrapper"> 
</div> 
</body>
</html>


Any help on this would be much appreciated.