When the next version of Horde_Imap_Client is released, which will 
probably be done at the same time IMP 5.0.15 is released.

I downloaded the latest git commit 
8c2a109930faddae9852434f7c475263cd70cd6b and replaced the appropriate 
files with
framework/Imap_Client/lib/Horde/Imap/Client/Search/Query.php  and
framework/Imap_Client/lib/Horde/Imap/Client/Socket.php
and et voilà - it works as expected! Thank you very much! When do you 
expect to release those fixes upstream so one could easily update via 
PEAR?
Best regards
Henri Wahl

Changes have been made in Git for this ticket:

Bug #10726: More thorough workarounds for bad search charsets
Store list of supported search charsets, if server reports it.

Attempt to convert text to different charset, if original charset was
rejected.

  2 files changed, 50 insertions(+), 16 deletions(-)
http://git.horde.org/horde-git/-/commit/bdafaffc9f122690dd951d04b7242cd20f52881f

Issue is that Exchange 2003 does not appear to support UTF-8 in SEARCH 
queries (which is kind of ridiculous in this day and age...)  RFC 3501 
only requires compliant servers to handle US-ASCII.

Although, in your situation, we are just searching by flags, so there 
is no reason to even add CHARSET information.  So I've made this change.
This is completely expected.  The THREAD extension REQUIRES the server 
to handle UTF-8 queries (which begs the question: the server can 
obviously handle UTF-8, so why does it now allow UTF-8 support in the 
default SEARCH command?)

This is probably a good idea to complete sanity checking in the search 
query when a bad charset is used.  The only option is to downconvert 
to us-ascii.

Changes have been made in Git for this ticket:

[mms] Only add CHARSET data to SEARCH queries if necessary (Bug #10726)

  3 files changed, 25 insertions(+), 7 deletions(-)
http://git.horde.org/horde-git/-/commit/8c2a109930faddae9852434f7c475263cd70cd6b

The following appears in normal horde log file when this mentioned 
error appears:

2011-11-09T22:27:53+01:00 ERR: HORDE [imp] IMAP error: Specified 
character set not supported. [pid 10427 on line 340 of 
"/var/www/horde/imp/lib/Imap.php"]
2011-11-09T22:27:53+01:00 DEBUG: HORDE  1. 
IMP_Ajax_Application->doAction() /var/www/horde/services/ajax.php:60
  2. Horde_Core_Ajax_Application->doAction() 
/var/www/horde/imp/lib/Ajax/Application.php:88
  3. call_user_func() /usr/share/pear/Horde/Core/Ajax/Application.php:95
  4. IMP_Ajax_Application->viewPort()
  5. IMP_Ajax_Application->_viewPortData() 
/var/www/horde/imp/lib/Ajax/Application.php:695
  6. IMP_Views_ListMessages->listMessages() 
/var/www/horde/imp/lib/Ajax/Application.php:1980
  7. IMP_Mailbox_List->getSortedList() 
/var/www/horde/imp/lib/Views/ListMessages.php:126
  8. IMP_Mailbox_List->_buildMailbox() 
/var/www/horde/imp/lib/Mailbox/List.php:608
  9. IMP_Imap->search() /var/www/horde/imp/lib/Mailbox/List.php:285
10. IMP_Imap->__call()

This log entry appears with every sorting option which is not "threads".

I attached the communication as dumped with the debug parameter in 
backends.php. In line 8 the error message appears which later gets 
displayed in IMP.
Things are getting funnier. I accidentally changed sort order in DIMP 
interface from subject to thread and suddenly the mails where listed 
as expected. Further tests showed that they only get listed when 
sorted by thread. Any clue?
Best regards
Henri Wahl

To further debug this issue, we need details of the IMP -> IMAP/POP 
communication.

To enable debugging, see instructions contained in 
imp/config/backends.php (the 'debug' config parameter).

Debugging should not be enabled on a production server,   Attach/post 
only the portion of the log that directly deals with the problem 
reported (it may be simplest to clear the log file and then perform 
the event that causes the error).

Hello,
I am writing her because after some days of searching I did not find a 
solution.
I am switching from Horde3/IMP4 to Horde4/IMP5. Transistion worked 
mostly great, only problem is that IMP5 (5.0.14) is not able to 
connect to an Exchange 2003 server via IMAP but was no problem with 
IMP4. When logging in, the folders get displayed on the left side of 
the window but no content is shown. Instead of content there is an 
error message claiming "Mailbox listing failed IMAP error: Specified 
character set not supported."
The old working servers.php has this entry:

$servers['exchange1'] = array(
     'name' => 'Exchange',
     'server' => 'exchange.ifw-dresden.de',
     'hordeauth' => false,
     'protocol' => 'imap/ssl/novalidate-cert',
     'port' => 993,
     'maildomain' => 'ifw-dresden.de',
     'smtphost' => 'mail.ifw-dresden.de',
     'smtpport' => 25,
     'realm' => '',
     'preferred' => '',
     'dotfiles' => false,
     'hierarchies' => array()
);

The new non-working backends.local.php this one:

$servers['exchange1'] = array(
     // Disabled by default
     'disabled' => false,
     'name' => 'Exchange',
     'hostspec' => 'exchange.ifw-dresden.de',
     'hordeauth' => false,
     'protocol' => 'imap',
     'port' => 993,
     'secure' => 'ssl',
     'maildomain' => 'ifw-dresden.de',
     'smtphost' => 'mail.ifw-dresden.de',
     'smtpport' => 25,
     'acl' => false,
     'cache' => false,
);

The Unix Dovecot server I also connect to works without any trouble. 
Could anyone help out?
Thank you and best regards

Henri Wahl