We just conducted an upgrade test to WordPress 3.9.1 on our development server and after the upgrade no one can log in except Netwrok Admins and even then there is a redirect issue.
If I attempt to log into any site (via /wp-admin) I am re-directed to the login.php and presented with the "Network Admins Only" warning. This is strange because I am a Network Admin. If I hit refresh I get to the dashboard. However if a non-network admin user attempts to login they get the same thing and the refresh trick does not work.