[imp] Login failed - Groupware Webmail Edition 1.1

[elias]

Hello list,

after I have used Horde Groupware Webmail Edition 1.0.x for a long time
without problems I have now made a fresh installation of the new Webmail
Edition 1.1 on the same server with the same environment (Ubuntu,
dovecot, postfix etc.)
Since that I always get "Login failed" when I try to login with my IMAP
username/password at IMP. (Login in Horde is possible!)

In horde.log I get:
HORDE [error] [imp] FAILED LOGIN for elias [85.0.81.136] to
{localhost:993 [imap/ssl/novalidate-cert]} [pid 5541 on line 139 of "/v
ar/www/elias/horde/imp/lib/Auth/imp.php"]

Here are some more details:

1. /horde/test.php and /imp/test.php don't show any problems (I can post
it here if useful)

2. "PHP Mail Server Support Test" (in /imp/test.php) gave me the
following successful configurations:

The following configurations were successful and may be used in your
imp/config/servers.php file:
    Configuration 1
        'server' => 'localhost'
        'protocol' => 'imap/notls'
        'port' => '143'

    Configuration 2
        'server' => 'localhost'
        'protocol' => 'imap/ssl/novalidate-cert'
        'port' => '993'

    Configuration 3
        'server' => 'localhost'
        'protocol' => 'imap/tls/novalidate-cert'
        'port' => '143'
 
3. I have tested all of these configurations in my servers.php (And I
used a copy of the servers.php from my old horde installation, which had
worked)

4. I also have Roundcube Mail, Iloha Mail, Atmail Open on my server and
with all of these applications can authenticate without problems

5. I can successfully login at my IMAP server with Telnet

6. I get no error in apache2 error.log

7. I get no message in IMAP log

8. In /horde/config/conf.php I have tried:
$conf['auth']['params']['imapconfig'] = 'separate';
$conf['auth']['params']['hostspec'] = 'localhost';
$conf['auth']['params']['port'] = 143;
$conf['auth']['params']['protocol'] = 'imap/notls';

and:
$conf['auth']['params']['imapconfig'] = 'dsn';
$conf['auth']['params']['dsn'] = '{localhost:143/imap/notls}';


Does anybody has an idea, what else I could do?

Thanks in advance
Elias