Horde (4.0.11) authentication failed after password reset

A fatal error has occurred
IMAP server denied authentication.

1. require() /srv/www/webmail/imp/index.php:19
2. Horde_Imap_Client_Base->openMailbox() /srv/www/webmail/imp/mailbox.php:86
3. Horde_Imap_Client_Base->login() /usr/share/php/Horde/Imap/Client/Base.php:808
4. Horde_Imap_Client_Socket->_login() /usr/share/php/Horde/Imap/Client/Base.php:609
5. Horde_Imap_Client_Base->_exception() /usr/share/php/Horde/Imap/Client/Socket.php:422

Details
The full error message is logged in Horde's log file, and is shown below only to administrators. Non-administrative users will not see error details.

Horde_Imap_Client_Exception Object
(
    [details] =>
    [message:protected] => IMAP server denied authentication.
    [string:Exception:private] =>
    [code:protected] => 102
    [file:protected] => /usr/share/php/Horde/Imap/Client/Base.php
    [line:protected] => 220
    [trace:Exception:private] => Array
        (
            [0] => Array
                (
                    [file] => /usr/share/php/Horde/Imap/Client/Socket.php
                    [line] => 422
                    [function] => _exception
                    [class] => Horde_Imap_Client_Base
                    [type] => ->
                    [args] => Array
                        (
                            [0] => IMAP server denied authentication.
                            [1] => LOGIN_AUTHENTICATIONFAILED
                        )

                )

            [1] => Array
                (
                    [file] => /usr/share/php/Horde/Imap/Client/Base.php
                    [line] => 609
                    [function] => _login
                    [class] => Horde_Imap_Client_Socket
                    [type] => ->
                    [args] => Array
                        (
                        )

                )

            [2] => Array
                (
                    [file] => /usr/share/php/Horde/Imap/Client/Base.php
                    [line] => 808
                    [function] => login
                    [class] => Horde_Imap_Client_Base
                    [type] => ->
                    [args] => Array
                        (
                        )

                )

            [3] => Array
                (
                    [file] => /srv/www/webmail/imp/mailbox.php
                    [line] => 86
                    [function] => openMailbox
                    [class] => Horde_Imap_Client_Base
                    [type] => ->
                    [args] => Array
                        (
                            [0] => IMP_Mailbox Object
                                (
                                    [cache] => Array
                                        (
                                            [n] => Array
                                                (
                                                    [0] => .
                                                    [4] => 1
                                                )

                                            [d] => Inbox
                                            [ro] =>
                                            [a] =>
                                        )

                                    [changed] => 0
                                    [_import:protected] =>
                                    [_mbox:protected] => INBOX
                                )

                            [1] => 2
                        )

                )

            [4] => Array
                (
                    [file] => /srv/www/webmail/imp/index.php
                    [line] => 19
                    [args] => Array
                        (
                            [0] => /srv/www/webmail/imp/mailbox.php
                        )

                    [function] => require
                )

        )

    [previous:Exception:private] =>
)