We have a zabbix-server (version 3.0.4) with several hosts (about 300), configured via web interface of zabbix. On each host there are several data items with the type "Zabbix agent", i.e. data items gather in the passive mode, with a certain periodicity on the basis of requests from the server to hosts with the agent, set on them. Data are gathered correctly.

There is a need to get optional data from one of such hosts by zabbix_sender.

The new data item with the type "Zabbix trapper" was configured via web interface of zabbix. If I call zabbix_sender at the host, which should send this data (zabbix_sender.exe -z 'address of zabbix server' -s 'host name' -p 'port of zabbix server' -k 'item key' - o 'new item value' -vv) - it works only when passive checks from the server to the agent (configured earlier) aren't executed. If I call zabbix_sender at the time of such check, I get the error "Connection timeout occured", and after check completion zabbix_sender begins to work again (data are sended to the server correctly).

The full text of error: zabbix_sender.exe [6988]: DEBUG: send value error: cannot connect to [[X.X.X.X]:10051]: A connection timeout occurred. Sending failed.

The port 10051 is open on the server, and we don't use any firewall. We see that it is related to the passive checks. Passive checks are configured to begin every 5 minutes. For example, next passive check will begin at 15:00. Before this moment (at 14:48, 14:49, etc.) zabbix_sender works well and returns results. At 15:00 it stops to work and returns error "Connection timeout occured". At 15:01 it begins to work, and at 15:05 it will stop working again. When we deactivate passive checks - zabbix_sender begins to work properly, without any errors.

When we use the command "netstat" at the moment of sender's failing we see that our host tries to connect to the server's port 10051, but does not get the answer (SYN_SENT status).

This problem is not only on one host. I have already got it on two different hosts. Other hosts I didn't check, but to my mind they will have the same problem.

Besides it, we tried to use active checks on this host (for elements with type "Zabbix agent (active)"). They work with passive checks simulaneously, but time to time we see the same error in the agent's log:
"3736:20170117:161538.336 active check configuration update from [X.X.X.X:10051] started to fail (cannot connect to [[X.X.X.X]:10051]: A connection timeout occurred.)
3736:20170117:161808.145 active check configuration update from [X.X.X.X:10051] is working again
3736:20170117:161931.076 active check data upload to [X.X.X.X:10051] started to fail ([connect] cannot connect to [[X.X.X.X]:10051]: A connection timeout occurred.)
3736:20170117:162015.178 active check data upload to [X.X.X.X:10051] is working again"

Could you explain, please, in what cases this error occurs and how to avoid it? It seems to be, that sometimes passive and active checks can't work on one host simultaneously. We have already checked a lot of settings (http://stackoverflow.com/questions/41599734/zabbix-zabbix-sender-and-passive-checks-based-on-zabbix-agent-do-not-work-sim), but could not resolve it.