Same problem here... I didn't touch the admin for a few days, tried to log in today and no success so far...
I cleared all cookies, cache and browser history, tried it in 3 different browsers and 2 different computers... No success so far...

What I did find with capturing the HTTP headers was that after the login I get redirected to the index of the admin ( /admin/index.php ) with a 302 and it sets a cookie ( zenAdminID=<hash>; path=/; domain=<full-domain>, ) at the same time.

On retrieving the index page the zenAdminID cookie is offered in the call. At the index I get redirected to the login page ( /admin/login.php ) with a 302.