This seems to happen from time to time with other people with exactly 
the same setup. Please check the mailing list archive.

This doesn't seem to be a Horde problem, and Horde 3.1.x is only 
supported for security fixes anymore anyway.

I'm using Horde 3.1.6 with IMP 4.1.6 and am using composite 
authentication to sidestep the MIMP (1.0.2) double login issue as 
recommended on this Horde Wiki page:



http://wiki.horde.org/MIMPHowTo



The only difference is that I'm authenticating Horde with a secure 
LDAP server, and IMP with an IMAP server, like this:



/* composite configuration for horde and mimp */

$conf['auth']['params']['drivers'] = array(

     'horde' => array('driver' => 'ldap',

                      'params' => array('hostspec' => 'ldaps://[hidden]',

                                        'basedn' => 
'ou=[hidden],dc=[hidden],dc=[hidden]',

                                        'version' => '3',

                                        'ad' => true,

                                        'uid' => 'uid',

                                        'encryption' => '[hidden]',

                                        'newuser_objectclass' => 
array('shadowAccount', 'inetOrgPerson'),

                                        'objectclass' => array('uid'),

                                        'filter_type' => 'objectclass',

                                        'password_expiration' => 'no'

                          )),

     'mimp' => array('driver' => 'application',

                     'params' => array('app' => 'mimp')));



$conf['auth']['params']['loginscreen_switch'] = '_horde_select_loginscreen';



if (!function_exists('_horde_select_loginscreen')) {

     function _horde_select_loginscreen()

     {

         require_once 'Horde/Browser.php';

         $browser = new Browser();

         if ($browser->isMobile()) {

             return 'mimp';

         }

         return 'horde';

     }

}



/* IMP servers.php configuration */

$servers['imap'] = array(

     'name' => 'IMAP Server',

     'server' => '[hidden]',

     'hordeauth' => true,

     'protocol' => 'imap/ssl/novalidate-cert',

     'port' => 993,

     'maildomain' => '[hidden]',

     'smtphost' => '[hidden]',

     'smtpport' => 25,

     'realm' => '',

     'preferred' => '',

);





The LDAP (AD v. 3) and IMAP servers contain the same usernames and 
passwords, because we're in the process of converting the mail server 
to LDAP authentication, so I have IMP configured to use Horde 
authentication (see 'hordeauth' line above).



All of this works fine most of the time, but occasionally users who 
access /imp directly get presented with the Horde left sidebar frame, 
indicating a successful LDAP login, and a login screen in the right 
frame, indicating an unsuccessful IMAP login. If they click on "Mail" 
in the left sidebar without logging in again, however, they are able 
to access their IMAP Inbox. It appears that either a) the login 
process isn't waiting for a successful login message from the IMAP 
server before reporting a login failure (i.e. a timeout is occurring), 
or b) the first IMAP login attempt actually does fail, for whatever 
reason, and Horde retries the IMAP login when a second attempt is made 
to access the user's IMAP mail, which subsequently succeeds.