So all of a sudden, after a week of using it, I get an error message on my zabbix server
gui (http://localhost/zabbix/.)
The error says: Zabbix serve
My problem was caused by having external ip in $ZBX_SERVER setting.
I changed it to localhost instead so that ip was resolved internally,
$sudo nano /etc/zabbix/web/zabbix.conf.php
Changed
$ZBX_SERVER = 'external ip was written here';
to
$ZBX_SERVER = 'localhost';
then
$sudo service zabbix-server restart
Zabbix 3.4 on Ubuntu 14.04.3 LTS