Was playing around with Sitecore 6.5.0 rev. 120427 and .NET 4.5, first thing that happen is I got this error.
Object of type 'System.Int32' cannot be converted to type 'System.Web.Security.Cryptography.Purpose'
it seems that this has been fixed in Sitecore CMS 6.5.0 rev. 120706 (6.5.0 Update-5), but for those who want to do workaround for it, here’s what you can do.
add this line to the web.config
<setting name="login.rememberlastloggedinusername" value="false" />