I had the same problem.
It’s weird because it didn’t manifest itself on the dev server, only on live... in the end I traced it down to /manager/media/browser/mcpuk/seturl_js_tinymce.inc
The supplied file had the following:
<script language="javascript" type="text/javascript" src="[+editor_path+]jscripts/tiny_mce/tiny_mce_popup.js"></script>
<script language="javascript" type="text/javascript" src="[+editor_path+]tinymce.modxfb.js"></script>
It should have been:
<script language="javascript" type="text/javascript" src="[+editor_path+]/jscripts/tiny_mce/tiny_mce_popup.js"></script>
<script language="javascript" type="text/javascript" src="[+editor_path+]/tinymce.modxfb.js"></script>
Note the forward slash after [+editor_path+].
Subtle, but it resolved the problem for me.
Chris