When I try it, the impersonation is working, it's just that their "Impersonation Start Page" setting on the bio block is set to redirect to the internal Rock home page and the person you are impersonating does not have access to that page. So as soon as you impersonate them, you get a security error. You can either just then go to the "myaccount" page, or change that setting to take them to the public "myaccount" page.