FCKEditor and 500 server image browser error in asp.net

The apparently XML derived “500 server error” problem with FCKEditor’s image upload tool and asp.net seems very common, and there seem to be half a million different confusing solutions on the web. In my case, it turned out the problem was paths and was fixed by adding a new key entry in web.config:

So nothing to do with XML at all really and the error was simply the result of an incorrect URL resulting in a 500 server error that returned invalid XML… A helpful error code could be added to FCKEditor, perhaps?