another startup problem

[Jeroen van Vlierden]

I have created the sample webservice HelloWorld.
It works now from the testpage


However, when I try to instantiate it from either a console app or a
windows forms app I get the following runtime exception

An unhandled exception of type 'System.MissingMethodException' occurred
in system.web.services.dll

Additional information: Method not found: Void
System.Xml.XmlTextReader.set_ProhibitDtd(Boolean).

I wonder what could be wrong here...