I have found a muuuuch easier more configurable way to get the 404 error to disappear when trying to use the FCKEditor... No need to recompile!
In the web.config at the top-level of the BugNET application directory, go down to the line with the tag
<HtmlEditorProvider defaultProvider="FckHtmlEditorProvider">
Then change the BasePath attribute in the subtag to ensure that there is a slash at the beginning of the path. This is counter-intuitive since in my web-structure, BugNET is not in the Root directory, but by making the path look like its the root directory seems to repair the problem.
So, change the following:
<add name="FckHtmlEditorProvider" type="BugNET.Providers.HtmlEditorProviders.FckHtmlEditorProvider, BugNET.Providers.FckHtmlEditorProvider" ... BasePath="FCKEditor/" />
To:
<add name="FckHtmlEditorProvider" type="BugNET.Providers.HtmlEditorProviders.FckHtmlEditorProvider, BugNET.Providers.FckHtmlEditorProvider" ... BasePath="/FCKEditor/" />
Please note the forward-slash at the beginning of the FCKEditor string in the second tag. That fixed it for me. Good luck.