Running on host that is NOT the zabbix server:

zabbix_sender -vv -c /etc/zabbix/zabbix_agentd.conf --key rclone.log --value "2020/05/05 09:44:14 test 123"
zabbix_sender [11163]: DEBUG: answer [\{"response":"success","info":"processed: 1; failed: 0; total: 1; seconds spent: 0.000112"}]
Response from "167.172.41.94:10051": "processed: 1; failed: 0; total: 1; seconds spent: 0.000112"
sent: 1; skipped: 0; total: 1

Running on host that IS the zabbix server:

zabbix_sender -vv -c /etc/zabbix/zabbix_agentd.conf --key rclone.log --value "2020/05/05 09:44:14 test 123"
zabbix_sender [14872]: DEBUG: answer [\{"response":"success","info":"processed: 0; failed: 1; total: 1; seconds spent: 0.000025"}]
Response from "127.0.0.1:10051": "processed: 0; failed: 1; total: 1; seconds spent: 0.000025"
sent: 1; skipped: 0; total: 1