I'm using the FCKEditor ASP.NET control on a web application that's deployed to a virtual directory. When my app is deployed to the root of a web site, FCKEditor works great. When I'm deployed in a virtual directory, I get an HTTP 404 error in the FCKEditor IFRAME. One quick look at the IFRAME properties shows that the URL is missing the virtual directory path. All prepared to hack on the FCKEditor.cs file, I discovered that some one else already thought of this scenario. I either overlooked a step in the documentation or this is an undocumented feature. Regardless, here is a universal solution to the problem... Just add the following key to the appSettings section of your web.config...
<add key="FCKeditor:BasePath" value="~/fckeditor/"/>
That's it! By default, FCKEditor uses /fckeditor/ and doesn't try to ResolveUrl to discover your virtual directory scenario. Only when you add this to your appSettings will FCKEditor look for the ~ and upon finding that will it run the path through ResolveUrl. That said, adding the above key to your appSettings should solve this problem for 99% of the people dealing with this issue.