IMCE upload window freezes (invalid JSON) when Theme Editor is enabled

When Theme Editor is enabled, the upload dialog window of IMCE freezes. The upload succeeds though, but one has to completely close the whole IMCE window in order to proceed with the new image or upoading other images.

I'm pasting below the firebug console error.

"NetworkError: 500 Service unavailable (with message) - http://example.site/imce?jsop=upload&dir=."
imce?j...d&dir=.
uncaught exception: Invalid JSON: <div id="skip-link"> <a href="#main-content" class="element-invisible element-focusable">Skip to main content</a> </div> <div id="page-wrapper"><div id="page"> <div id="header"><div class="section clearfix"> <div id="name-and-slogan" class="element-invisible"> <div id="site-name" class="element-invisible"> <strong> <a href="/" title="Home" rel="home"><span>example.site</span></a> </strong> </div> <div id="site-slogan" class="element-invisible"> Taking action on the root causes of climate change </div> </div> <!-- /#name-and-slogan --> </div></div> <!-- /.section, /#header --> <div id="main-wrapper"><div id="main" class="clearfix"> <div id="content" class="column"><div class="section"> <a id="main-content"></a> <h1 class="title" id="page-title">Error</h1> The website encountered an unexpected error. Please try again later. <div id="messages"><div class="section clearfix"> <div class="messages error"> <h2 class="element-invisible">Error message</h2> <em class="placeholder">Recoverable fatal error</em>: Object of class stdClass could not be converted to string in <em class="placeholder">DatabaseStatementBase-&gt;execute()</em> (line <em class="placeholder">2139</em> of <em class="placeholder">/var/aegir/platforms/drupal-7.12/includes/database/database.inc</em>).</div> </div></div> <!-- /.section, /#messages --> </div></div> <!-- /.section, /#content --> </div></div> <!-- /#main, /#main-wrapper --> </div></div> <!-- /#page, /#page-wrapper -->