After installing and configuring the zabbix plugin connection attempts are rejected:
root@zabbix:~# zabbix_get -s 10.0.0.1 -k agent.ping
zabbix_get [2925]: Check access restrictions in Zabbix agent configuration
This is due to zabbix not reloading the configuration after saving and applying in the web interface. Also on the Dashboard it is not possible to actually stop and start the service despite the buttons being shown.
It is possible however to restart it from the shell, which immediately applies the configured changes:

@thunfischbrot Thanks for your report! Which version of OPNsense and of os-zabbix-agent are you running?
I apologize for this omission, should have mentioned that they are all up-to-date:
FreeBSD 11.1-RELEASE-p6 6621d681e(stable/18.1)
OPNsense 18.1.2-efa2c4f74 [18.1.1-a5575d7bd] LibreSSL 2.6.4 (amd64)
Plugins os-arp-scan-1.1 os-iperf-1.0 os-wol-1.0_1 os-zabbix-agent-1.2
FWIW, try to update. 18.1.2 had a buggy service handling and you want to double check on 18.1.2_2.
On 9. Feb 2018, at 09:54, thunfischbrot notifications@github.com wrote:
I apologize for this omission, should have mentioned that they are all up-to-date:
FreeBSD 11.1-RELEASE-p6 6621d681e(stable/18.1) OPNsense 18.1.2-efa2c4f74 [18.1.1-a5575d7bd] LibreSSL 2.6.4 (amd64) Plugins os-arp-scan-1.1 os-iperf-1.0 os-wol-1.0_1 os-zabbix-agent-1.2—
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Just upgraded, still the same behaviour.
User-Agent Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0
FreeBSD 11.1-RELEASE-p6 6621d681e(stable/18.1)
OPNsense 18.1.2_2-43d2878aa [18.1.2-efa2c4f74] LibreSSL 2.6.4 (amd64)
Plugins os-arp-scan-1.1 os-iperf-1.0 os-wol-1.0_1 os-zabbix-agent-1.2

Hello, it's same with OPNsense 18.1.5-amd64 FreeBSD 11.1-RELEASE-p8 LibreSSL 2.6.4
I have to disable and re-enable the agent to restart it from the web interface
Found and fixed, thanks for the video!
Most helpful comment
Found and fixed, thanks for the video!