Problem

The Zabbix Web Service doesn't start automatically and has to be started manually (documentation). Even if I start it manually, I have to start it again after every reboot.

The documentation doesn't mention, I have to add the option -c to get it to work:

zabbix_web_service -c /etc/zabbix/zabbix_web_service.conf 

I found this forum thread (my post about how I did work around the issue):

https://www.zabbix.com/forum/zabbix-troubleshooting-and-problems/429591-scheduled-reports-cannot-connect-to-web-service-couldn-t-connect-to-server?p=443124#post443124

Possible Solution

Provide a systemd service unit file which runs

zabbix_web_service -c /etc/zabbix/zabbix_web_service.conf

at boot time.