Hi,
I would use Firebug and specifically the network tab to review requests for individual items. The editors tend to load additional javascript files for various plugins and possibly there are files that have bad permission or are not getting loaded for some reason.
I would also determine what version of the editor is on the page by viewing the source you should see the version as part of the url for the main editor javascript.
If the installation has been upgraded over time, you could potentially revert to older versions of the editor which probably still live on disk under /ClientScript
The version used is configured from Web.config like this:
<add key="CKEditor:BasePath" value="~/ClientScript/ckeditor362/"/>
so if you still have older versions of the editor you could try pointing to one of them and see if the problem happens in older versions or not.
I tend to always upgrade to new versions of the editors as soon as they come out but like any software there can be new bugs or regressions in new versions even though most new versions report that they fixed a lot of bugs. I think the guys who make CKeditor and TinyMCE have a difficult task trying to essentially make a word processor in javascript and keep up with all the changes in web browsers and web standards.
It would be good to isolate the problem definition as much as possible in terms of editor version, language used (either the browser preference or what is forced by configuration), which browser(s)/versions of browsers are affected, which skins work and which do not.
Best,
Joe