[horde] Configuring Horde initial application

[garnier@embratel.com.br]

I´m trying to configure IMP (v 4.0.3) as Horde (v 3.0.5) initial
application, I mean, I would like to go to IMP as soon as I log in to
Horde. I configured Horde to use IMP authentication (horde/config/conf.php:
$conf['auth']['driver'] = 'application' and $conf['auth']['params']['app']
= 'imp'). Then, I changed $_prefs['initial_application'] in
horde/config/prefs.php like this:

$_prefs['initial_application'] = array(
    'value' => 'imp',
    'locked' => true,
    'shared' => true,
    'type' => 'select',
    'desc' => sprintf(_("What application should %s display after login?"), $GLOBALS['registry']->get('name'))
);

But when I log in, Horde shows its initial page (some icons on top and a "last login" message on the middle). IMP is authenticated, but I have to
click IMP icon to go to it.
What am I doing wrong?