I need to add reference to \'System.XML.dll\' assembly into web.config (in order to try solution for problem, mentioned here: Problem with Extension method: IXmlLineInfo).
Try 'gacutil /l', and make sure you put in your web.config the one you really want to reference, depending on the target .NET release (e.g., 3.5, 4.0, etc). (In code terms, make sure you can load the assembly through Assembly.Load().)