Have you tried logging in with MxAdmin and then switching to a demo account?
The credentials for MxAdmin are in the project module > security > Administrator tab. Try logging in with this account and then using the mx demo user switcher (its the button above the feedback item) to switch to the demo user that you want.
Hope this helps!
James,
I have seen a similar behavior when there was an error on a page in my Mendix app. A few ideas for how to troubleshoot this:
Hope that will help you narrow down the cause of this.
Mike