[imp] Error subscribing to folder

[donnieg@cumberlandcollege.edu]

Hi,

First the particulars, I am using: IMP3.1, Horde 2.1,Turba 1.1, and UW IMAP.
 Here is a section from my horde/imp/config/servers.php file.

$servers['imap'] = array(
    'name' => 'IMAP Server',
    'server' => 'server.domainname.edu',
    'protocol' => 'imap/notls',
    'port' => 143,
    'folders' => 'mail/',
    'namespace' => '',
    'maildomain' => 'domainname.edu',
    'smtphost' => 'server.domainname.edu',
    'realm' => 'server.domainname.edu',
    'preferred' => ''
);

and a section from my horde/imp/config/prefs.php file
$_prefs['subscribe'] = array(
    'value' => 1,
    'locked' => true,
    'shared' => false,
    'type' => 'checkbox',
    'desc' => _("Use IMAP folder subscriptions")
);


When adding a folder, the folder is created in the mail folder but the 
following message is displayed:

"Warning: SUBSCRIBE failed: Can't append to subscription database (errflg=2) in 
Unknown on line 0"

Then when you try to subscribe to the folder, the following error message is 
displayed:

"Warning: SUBSCRIBE failed: Can't append to subscription database (errflg=2) in 
Unknown on line 0"

Can anyone give any advice on how I can solve the problem?
Thanks,
Donnie