FCKeditor中的XML request error (500)错误，并不仅仅是在没有目录的写权限时出现这种错误

一天，打开网站的后台，发表一个文章，上传图片时出现XML request error  (500)错误，查了一个资料，都说是因为没有目录的写权限。可我怎么不配置权限也不行。我查看了一个FCKeditor控件原代码,在FileWorkerBase.cs文件中有以下一段

protected string UserFilesPath
        {
            get
            {
                if ( sUserFilesPath == null )
                {
                    // Try to get from the "Application".
                    sUserFilesPath = (string)Application["FCKeditor:UserFilesPath"] ;

                    /**///// Try to get from the "Session".
                    //if ( sUserFilesPath == null || sUserFilesPath.Length == 0 )
                    //{
                    //    sUserFilesPath = (string)Session["FCKeditor:UserFilesPath"] ;
                        
                        // Try to get from the Web.config file.
                        if ( sUserFilesPath == null || sUserFilesPath.Length == 0 )
                        {
                            sUserFilesPath = System.Configuration.ConfigurationSettings.AppSettings["FCKeditor:UserFilesPath"] ;
                            
                            // Otherwise use the default value.
                            if ( sUserFilesPath == null || sUserFilesPath.Length == 0 ) 
                                sUserFilesPath = DEFAULT_USER_FILES_PATH ;

                            // Try to get from the URL.
                            if ( sUserFilesPath == null || sUserFilesPath.Length == 0 ) 
                            {
                                sUserFilesPath = Request.QueryString["ServerPath"] ;
                            }
                        //}
                    }

                    // Check that the user path ends with slash ("/")
                    if ( ! sUserFilesPath.EndsWith("/") )
                        sUserFilesPath += "/" ;
                }
                return sUserFilesPath ;
            }
        }

我把这一段注释掉后重新编译，就不出错了
//if ( sUserFilesPath == null || sUserFilesPath.Length == 0 )
                    //{
                    //    sUserFilesPath = (string)Session["FCKeditor:UserFilesPath"] ;