[horde] Missing spam / ham button within IMP

[sascha at schmidt.ps]

Hi,

I cannot figure out why the spam/ham button is missing within the imp  
webinterface. My configuration within the file backend.local.php:

<?php
$servers['imap']['disabled'] = true;
$servers['advanced'] = array(
     'disabled' => false,
     'name' => 'IMAP Server',
     'hostspec' => 'localhost',
     'hordeauth' => full,
     'protocol' => 'imap',
     'port' => 143,
     'secure' => 'tls',
     'spam' => array(
         'innocent' => array(
             'display' => true,
             'email' => null,
             'email_format' => 'digest',
             'null' => true,
             'program' => null
         ),
         'spam' => array(
             'display' => false,
             'email' => null,
             'email_format' => 'digest',
             'null' => true,
             'program' => null
         )
     ),
     'special_mboxes' => array(
         IMP_Mailbox::MBOX_DRAFTS => 'Drafts',
         IMP_Mailbox::MBOX_SENT => 'Sent',
         IMP_Mailbox::MBOX_SPAM => 'Junk',
         IMP_Mailbox::MBOX_TEMPLATES => 'Templates',
         IMP_Mailbox::MBOX_TRASH => 'Trash',
     )
);
?>

Additionally I've configured the Spam folder within my individual  
preferences. The Spam folder appears in the left folderlist and is  
locally named "Spam" ("Junk" called on IMAP).

But the spam/ham button is still missing.
Any hints for me?

Greets
Sascha