It definitely has to be a configuration issue not a bug in mojoPortal.
The only way IIS is going to serve a .aspx page for download is if the folder or specific files is somehow not configured for ASP.NET to handle them. Sounds like something must be very funky about the configuration on the /Admin folder or some specific files in there. I guess since the admin menu itself works it must be something more specific than the folder level settings, but for some reason those specific files are being seen as not handled by ASP.NET therefore IIS is just treating them as files and serving them up instead of letting ASP.NET handle the request for them. Very strange indeed. Maybe some strange IIS module settings involved.
The mojoPortal log is at /Data/currentlog.config, you might also look in the IIS logs, but especially at the IIS configuration on those particular files.
Hope that helps,