[imp] IMAP Toolkit Crash

[Seb Belcher]

Hi folks,

I'm hoping someone can help shed some light on a problem I am having 
with IMP.

Horde Version: 3.1.7
Imp Version: H3 (4.1.6)
Apache Version: Apache/2.0.55
PHP Version: 5.1.2

I set up Horde through the setup script with no major problems.  
Initially I tried running with POP3 authentication and this worked, I 
was able to get into the Admin screens and edit the config, all looks 
good on that front. 
But then I tried switching to IMAP.

Initially I ran the IMP test.php script and tried the credentials for my 
account on the local IMAP server (courier) the result is as follows:
**

    * /Trying protocol imap/notls, Port 143:/

          SUCCESS - INBOX has 1 messages (1 new 0 recent)

    * /Trying protocol imap/tls/novalidate-cert, Port 143:/

          SUCCESS - INBOX has 1 messages (1 new 0 recent)

All looking good, so I use the following in server.php:

$servers['imap'] = array(
    'name' => 'IMAP Server',
    'server' => 'localhost',
    'hordeauth' => false,
    'protocol' => 'imap/notls',
    'port' => 143,
    'maildomain' => 'my_domain.com',
    'smtphost' => 'localhost',
    'smtpport' => 25,
    'realm' => '',
    'preferred' => '',
);

Now I try to login to Horde again, using the exact same user/pass as in 
the test script, the login screen appears OK but when I click "login" 
and I am presented with:
    "Internal Server Error, this is an error with your script, check 
your error log for more information."

So, logs.

Apache gives:
[Wed Apr 30 23:58:14 2008] [error] [client 82.28.1.181] Premature end of 
script headers: redirect.php, referer: http://www.my_domain.com/horde/imp/

Syslog gives:
Apr 30 23:58:14 ds5402 php5-cgi: IMAP toolkit crash: Unable to look up 
user name

I've seen several threads about the visibility of /etc/passwd and I have 
checked it is there.  However, my mail users are not real users, they 
are all virtual users on virtual domains.  Is this what is causing the 
problem? Is there a way around it?
The main thing I don't understand is why the test script is able to 
authenticate and view my INBOX but IMP itself is causing this error?

Thanks in advance for any help.

Regards,
Seb.