==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.99
- Linux/BSD distribution name and version: CentOS 7.6
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Apache 2.4.6
- Manage mail accounts with iRedAdmin-Pro? Yes:  3.4
====

Error in pop up message box:
Could not save new password. Encryption function missing.

/var/log/httpd/access_log    (Don't appear to be errors written when this happens)
::1 - - [30/Apr/2019:23:30:22 -0400] "OPTIONS * HTTP/1.0" 200 - "-" "Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips mod_wsgi/3.4 Python/2.7.5 (internal dummy connection)"
::1 - - [30/Apr/2019:23:30:36 -0400] "OPTIONS * HTTP/1.0" 200 - "-" "Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips mod_wsgi/3.4 Python/2.7.5 (internal dummy connection)"

relevant Contents of [...]/roundcubemail/plugins/password/config.inc.php

// Path for dovecotpw/doveadm-pw (if not in the $PATH).
// Used for password_algorithm = 'dovecot'.
$config['password_dovecotpw'] = '/usr/bin/doveadm pw';

If you need more info -- please let me know -- I'm scratching my head here.