From FCKeditor Docs

In some IIS 6 instalations you might receive an error 'ASP 0131'.
That's due to a security setting in IIS that doesn't allow to use a relative paths in include files.

There are two solutions for this problem.

1. Using the Administrative tools you can change the setting as explained in MS KB Q332117
2. If you don't have access or don't want to change that, then open the ckfinder/core/connector/asp/connector.asp file, go to line 51 (it might change in newer versions) and change
   

<! -- #INCLUDE file="../../../config.asp" -->

with

<! -- #INCLUDE VIRTUAL="/ckfinder/config.asp" -->

Please, note that if you have placed ckfinder under a different root folder you'll need to adjust the path in that line

Also, there's a space in the previous lines due to a security restriction in this server, the lines should start with <!-- with no extra space between ! and -