Maybe its related, possibly you need to change the path, in Web.config
by default its configured like this:
<forms name=".mojochangeme" protection="All" timeout="50000000" path="/" cookieless="UseCookies" />
where the path / represents the root of the site, if you have another app at the root and mojoportal runing in a folder maybe changing it to /foldername would help.
I'm really not sure if that will help or not.