The Zabbix agent stopped sending active checks because it was stuck executing a script defined as a UserParameter, despite the fact that the Timeout parameter was set to 30 seconds.
There was an error in the script causing it to run indefinitely without timing out. My understanding is that the agent is supposed to timeout and exit even if the script itself does not exit.
The problem was solved by just fixing the hanging script. We did not have to restart the agent.
Please let me know what further information you need to investigate the issue.
Operations & IT engineer