[horde] Composite auth driver woes

[Chris St. Pierre]

I'm trying to get a composite authentication driver working.  My
config is as follows:

$conf['auth']['driver'] = 'composite';
$conf['auth']['params']['drivers']['imp_auth']['driver'] = 'application';
$conf['auth']['params']['drivers']['imp_auth']['params']['app'] = 'imp';
$conf['auth']['params']['drivers']['ldap_users']['driver'] = 'ldap';
$conf['auth']['params']['drivers']['ldap_users']['params'] =
   array('hostspec'   => 'ldap.nebrwesleyan.edu',
   'basedn'     => 'ou=people,o=nebrwesleyan.edu,o=isp',
   'version'    => '3',
   'ad'         => false,
   'uid'        => 'uid',
   'encryption' => 'ssha',
   'newuser_objectclass' => array('shadowAccount', 'inetOrgPerson'),
   'objectclass'         => array('nwuPerson'),
   'filter_type'         => 'objectclass',
   'password_expiration' => 'no');
$conf['auth']['params']['admin_driver'] = 'ldap_users';
$conf['auth']['params']['loginscreen_switch'] = '_horde_select_loginscreen';
if (!function_exists('_horde_select_loginscreen')) {
   function _horde_select_loginscreen() {
     return 'imp_auth';
   }
}

When I try to log in, I just get sent back to the login screen without
an error.  In the logs, I actually get "Login success" messages.

The really strange thing is that I started twiddling with this when I
noticed a typo; I used to have:

$conf['auth']['params']['admin_driver'] = 'ldap';

No driver named 'ldap' existed in my params.  But it worked!  (User
listing didn't work, obviously, but logins did.)  So apparently the
error condition of no admin_driver existing was "better" than the
error condition of ... whatever is wrong now.

Can anyone suggest what might be wrong?  Thanks!

Chris St. Pierre
Unix Systems Administrator
Nebraska Wesleyan University