[imp] no mailbox contents displayed - Bad IMAP request: Command Error. 10

[D G Teed]

I've asked colleagues to try out the IMP 5 and horde 4 webmail.
Most are reporting they like it and it works very well.

One reports an error and cannot see his mailbox.  Initially the mailbox
was showing messages, but then he clicked on Date to sort it
opposite of the default. Since then he cannot see anything in the
inbox, although the interface knows how many messages are
there and can display in the portal the latest unseen messages.

In traditional mode only, we also get an IMAP error. It corresponds
to what I see in the horde.log:

2011-08-19T10:58:21-03:00 ERR: HORDE [imp] Bad IMAP request: Command
Error. 10 [pid 3513 on line 340 of
"/usr/local/www/horde/horde-webmail-4.0/imp/lib/Imap.php"]

This is with MS Exchange.  The user had about 8000 messages.
I moved my own 8000+ messages to my own account
on Exchange and could not reproduce this issue.

Here is the client definition from the backend:

$servers['exchange'] = array(
    // ENABLED by default
    'disabled' => false,
    'name' => ' Exchange Email Server',
    'hostspec' => 'exchange.example.com',
    'hordeauth' => false,
    'protocol' => 'imap',
    'port' => 143,
    // Plaintext logins are disabled by default on IMAP servers (see RFC 3501
    // [6.2.3])
    'secure' => 'tls',
    'maildomain' => 'example.com',
    // 'smtphost' => 'smtp.example.com',
    // 'smtpport' => 25,
    'smtphost' => 'bellatrix2.example.com',
//    'quota' => array (
//       'driver' => 'imap',
//      'params' => array(
//          'hide_when_unlimited' => true,
//            'unit' => 'MB'
//       )
//    ),
    'cache' => false,
    'preferred' => ''
);


(example.com placeholder for actual domain)

What is recommended to trace this further, or is this a known problem?