Well, I think that very likely could be the problem why it can't set the cookies.
I did see the openid error when I try to visit your /Secure/UserProfile.aspx page. I'm looking into a solution for that now.
I can create a medium trust environement on my machine by using this in system.web section of web.config:
<trust level="Medium" originUrl="" />
However when I do this I cannot use MySql connector so I don't know how it works on your host unless they are using some custom level of security and not the default medium trust. Or maybe its because my dev machine is Vista/IIS7 and things might be more strict in IIS7.
Joe