To get the Roundcubemail password plugin to work properly, so you can change passwords, do the following:

cd /var/www/roundcubemail/plugins/password
cp config.inc.php.dist config.inc.php
For nginx:
chown nginx config.inc.php
For apache:
chown apache config.inc.php

chmod 0640 config.inc.php

Then edit /var/www/roundcubemail/plugins/password/config.inc.php and make the following changes:

You can change other fields that you would like to change, but these changes below should 
hopefully get things working.

Change:
$config['password_algorithm'] = 'clear';
To:
$config['password_algorithm'] = 'dovecot';

Change:
$config['password_dovecotpw'] = '/usr/local/sbin/dovecotpw';
To:
$config['password_dovecotpw'] = '/usr/bin/doveadm pw';

Change:
$config['password_dovecotpw_method'] = 'CRAM-MD5';
To:
$config['password_dovecotpw_method'] = 'SSHA512';

Change:
$config['password_dovecotpw_with_method'] = false;
To:
$config['password_dovecotpw_with_method'] = true;

Change:
$config['password_db_dsn'] = '';
To:
$config['password_db_dsn'] = 'mysql://vmail:Mysql-vmail-database-password@localhost/vmail';
The Mysql vmail database password is in the /root/SlackerMail/SlackerMail.setup file.

Change:
$config['password_query'] = 'SELECT update_passwd(%P, %u)';
To:
$config['password_query'] = 'UPDATE mailbox SET password=%P WHERE username=%u LIMIT 1';

That should do it. Please let me know how it works out.
Thanks,
Wayne
wjack@the-slacker.com