The new FCKeditor version 2.6 seems to have a bug which for me was quite hard to track down.
I use PHP to interface with it, so the bug which causes images uploaded via the quickuploader is found in the
file on line 291.
The error reported by firebug is: uncaught exception: Permission denied to get property Window.OnUploadCompleted
Simply change the following line:
var test = window.top.opener.document.domain ;
var test = window.parent.OnUploadCompleted ;
Here's the direct link
to the patch.
Hope that helped.