Most likely you are running another .NET app in the root and running mojoportal in a virtual directory beneath it. In this case a machine key defined in the root level app interferes with the mojoportal one. There are ways around that kind of issue by wrapping the system.web section in a location element. Do a little searching and reading about the location elelment in web.config if that is your situation.
Its also possible that your machine.config does not allow machine key in web.config if some strict policy is being enforced.
Hope it helps,