Hi Dave,
Nothing from your log gives me any clues to help. In medium trust it is expected to log that error every time the application starts so there is nothing unusual.
I would make sure you are running the same version of mojoportal in the root as you are in the 4 child sites. And when you upgrade always make sure to use the new Web.config and transfer any customizations to it, don't try to keep the old one as it may not have all the latest settings.
Best,
Joe