On a new installation of silverstripe 2.4alpha, the admin login was working with the default tutorial theme, but is not working with a custom theme.
The link in the login form is missing an absolute path slash, so the form action is set to "Security/Login"...
This means users cannot log in, because instead of going to /Security/Login, it tries to load Security/Security/Login which doesn't exist.
I'm not sure what the best way to fix this is...