==== Required information ====
- iRedMail version (check /etc/iredmail-release):
- Linux/BSD distribution name and version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):
- Web server (Apache or Nginx):
- Manage mail accounts with iRedAdmin-Pro?
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Hello All,
Please pardon me for my lack of knowledge.  I just installed the current version of iRedMail on RHEL 7.  I'm able to login to  iredadmin successfully.  However, when I try to log into roudcube webaccess, I get the below error.  Any thoughts on what I could be doing wrong, or what I could have configured improperly?  Thanks.

This page isn’t working
example.com is currently unable to handle this request.
HTTP ERROR 500

This is the output from log file:
May 24 11:20:07 mail roundcube: PHP Fatal error:  Call to undefined function mb_strtolower() in /var/www/roundcubemail-1.2.4/program/include/rcmail.php on line 584

This is what's on lines 580 thru 584:

// Convert username to lowercase. If storage backend
        // is case-insensitive we need to store always the same username (#1487113)
        if ($login_lc) {
            if ($login_lc == 2 || $login_lc === true) {
                $username = mb_strtolower($username);