I think I may have the same problem, but it looks like the iRedMail setup has applied some configuration to this, using method “dovecot”.

Some Dovecot releases have bug, when running "doveadm pw" command, it tries to read ssl cert, but it doesn't have required privilege to read cert, hence command failed.

If you're running such Dovecot releases, please switch password_algorithm to, e.g. "SSHA512".

Can that not work, and I need to use the settings here?

Did you create a specific additional MySQL user for this?

$config['password_algorithm'] = 'ssha512';
$config['password_algorithm_prefix'] = '{SSHA512}';
$config['password_db_dsn'] = 'mysql://vmailadmin:vmailadminpassword@localhost/vmail';
$config['password_query'] = 'UPDATE `mailbox` SET `password` = %P, modified=now() WHERE `username` = %u LIMIT 1';

==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): latest
- Deployed with iRedMail Easy or the downloadable installer?
- Linux/BSD distribution name and version: Ubuntu 18.04 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Apache2
- Manage mail accounts with iRedAdmin-Pro? no
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Can anybody point me to the proper config for the Roundcube password plugin to work with iRedMail?

I'm currently getting the following error...

DB Error: [1305] FUNCTION roundcube.update_passwd does not exist (SQL Query: SELECT update_passwd('someHash', 'user@domain.com')) in /opt/www/roundcubemail-1.5.3/program/lib/Roundcube/rcube_db.php on line 566 (POST /mail/?_task=settings&_action=plugin.password-save)

Well, I figured it out.  Not sure if this is an 'approved' way to do this or not.  Making these entries in the roundcubemail/plugins/password/config.inc.php file makes this functional.

$config['password_algorithm'] = 'ssha512';
$config['password_algorithm_prefix'] = '{SSHA512}';
$config['password_db_dsn'] = 'mysql://vmailadmin:vmailadminpassword@localhost/vmail';
$config['password_query'] = 'UPDATE `mailbox` SET `password` = %P, modified=now() WHERE `username` = %u LIMIT 1';

vmailadmin being an iRedMail user and only user allowed to update records in the vmail database.

This works and I offer no warranty.  As far as making this a little more secure/better it might be best to create another user that only has update permission on the mailbox table, or allowing the roundcube database user this permission.

If anyone else has any feedback I would love to hear it.

Can anybody point me to the proper config for the Roundcube password plugin to work with iRedMail?

I'm currently getting the following error...

DB Error: [1305] FUNCTION roundcube.update_passwd does not exist (SQL Query: SELECT update_passwd('someHash', 'user@domain.com')) in /opt/www/roundcubemail-1.5.3/program/lib/Roundcube/rcube_db.php on line 566 (POST /mail/?_task=settings&_action=plugin.password-save)