Hi Andrew,
Did you at any time deploy your application to an IIS server to another subfolder than the root ?
I'm trying to test on an IE7, which I only have on a Virtual Machine, but I think I might have found some issue here which I will explore further and report shortly.
Anyway, judging from the application attached to the issue, then that application is using FCKLibrary, which has not been updated for a very long time. Have you tried using the FCKEditor from Gizmox.WebGui.Forms.Editors, which is inside the Extended Dll ?
In any case, please email your current version of the project to support so I will have the exact same version as you are using.
Palli