ok, if the upper web.config has that then maybe commenting out the machine key in mojoportal web.config will solve it. Also maybe not though, for best results I would make mojoportal a root web site of its own. I can't predict what settings are affected by a web.config file higher up the chain.
If you remove the upper web.config might solve it too, is it needed for something else or just leftover from something else.
Hope it helps,
Joe