the fix for bug
ZBX-448 (https://support.zabbix.com/browse/ZBX-448) introduced a new problem while using external checks. When the zabbix server stops for whatever reason while the external checks are still running the external checks aren't stopped. (maybe this is also an bug?). But the external checks are assigned to the listening socket of the zabbix server.
The problem is that the check introduced in
ZBX-448 only checks if there is something listening to the right port, not if it responds. This can be easily checked by not only connecting but also sending en request to the server en waiting for the response.