[turba] Set turba as the default ( initial_application ) after login

[Feczak Szabolcs]

I have tried to set

$_prefs['initial_application'] = array(
    'value' => 'turba',
    'locked' => true,
    'shared' => false,
    'type' => 'select',
    'desc' => _("What application should Horde display after login?"));

in /horde/config/prefs.php

but after login I have got an empty frame with motd and
horde menu in the bottom frame.

What else should I do ?