Zabbix Server 500 error

[netfl94]

I've got a new install of Zabbix running on Ubuntu 16.04 that's giving me a 500 error on the web server. Everything was working as it should, then I went to comment out "mibs:" in /etc/snmp/snmp.conf and restarted the server. It was initially throwing CacheSize errors, and I've increase that from 128M to 4096M. That error has since disappeared in the log fines, however the web interface still keeps showing a 500 error.

Only error I can find is the following in apache error.log:

[Fri Mar 10 07:54:57.929315 2017] [:error] [pid 18316] [client 10.2.15.109:54036] PHP Fatal error: Uncaught Error: Call to u
ndefined function mb_strlen() in /usr/share/zabbix/include/classes/user/CProfile.php:330\nStack trace:\n#0 /usr/share/zabbix/
include/classes/user/CProfile.php(214): CProfile::checkValueType('zabbix.php', 3)\n#1 /usr/share/zabbix/include/menu.inc.php(
393): CProfile::update('web.menu.view.l...', 'zabbix.php', 3)\n#2 /usr/share/zabbix/app/views/layout.htmlpage.php(34): zbx_co
nstruct_menu(Array, Array, Array, 'dashboard.view')\n#3 /usr/share/zabbix/app/views/layout.htmlpage.php(126): local_generateH
eader(Array)\n#4 /usr/share/zabbix/include/classes/mvc/CView.php(171): include('/usr/share/zabb...')\n#5 /usr/share/zabbix/in
clude/classes/core/ZBase.php(398): CView->getOutput()\n#6 /usr/share/zabbix/include/classes/core/ZBase.php(156): ZBase->proce
ssRequest(Object(CRouter))\n#7 /usr/share/zabbix/include/config.inc.php(33): ZBase->run('default')\n#8 /usr/share/zabbix/zabb
ix.php(21): require_once('/usr/share/zabb...')\n#9 {main}\n thrown in /usr/share/zabbix/include/classes/user/CProfile.php on
line 330