I recently had an issue with one of my Umbraco sites where I just couldn't log in to the dashboard on my development machine. Firing up Fiddler I found Umbraco was logging me in but instantly logging me back out again by redirecting me to /umbraco/logout.aspx.
<httpCookies requireSSL="true" />
A quick web.config transform so this is set to false on localhost saved the day. Huzzah!