The FCKeditor worked fine on my drupal-site (5.7) until I uploaded a .swf and display it on the page.
But now the FCKeditor does not show-up anymore on other entries on the page.
Note. I do not understand this exception, but i can see that i get it from the embedded .swf when I use the redirected URL http://www.mysite.com, when I am directly on 122.79.233.122/ the exception does not happen.
It seems that this exception prevents fckeditor from starting.

Re: FCKEditor does not show in Drupal, due to SWF error on page
I didn't bother to debug it because it didn't cause any further problem, but my guess at that time was that the problem lied in the swfobject integration. If you are using it, you can try to update as they released very recently the version 2.0 and maybe they have addressed this problem.