Active Agent cannot obtain list of active checks

[McThump]

Can anyone suggest how to resolve or troubleshoot this puzzling problem?

The Zabbix agent (Win64 5.2.3) is discovered by the server but cannot receive the list of active checks. I have hundreds of computers at about 20 different locations running this agent, but only 2 exhibit this problem. These 2 computers are on different networks at different locations.

Zabbix server version: 5.2.3 running on CentOS
Zabbix agent version: Win64 5.2.3
Not using a proxy
I am able to telnet from the host to the Zabbix server port 10051

zabbix_server.log
1147:20210917:214330.710 cannot send list of active checks to "73.223.51.63": host [GED131] not found

zabbix_agentd.log
5892:20210918:182540.065 In send_buffer() host:'zabbix.ait.cx' port:10051 entries:0/100
5892:20210918:182540.066 End of send_buffer():SUCCEED
5892:20210918:182540.068 In refresh\_active\_checks() host:'zabbix.ait.cx' port:10051
5892:20210918:182540.108 In zbx\_tls\_connect(): psk_identity:"ZAPSK"
5892:20210918:182540.109 zbx\_psk\_client_cb() requested PSK identity "ZAPSK"
5892:20210918:182540.142 End of zbx\_tls\_connect():SUCCEED (established TLSv1.3 TLS\_CHACHA20\_POLY1305_SHA256)
5892:20210918:182540.143 sending \[{"request":"active checks","host":"GED131"}\]
5892:20210918:182540.145 before read
5892:20210918:182540.193 End of refresh\_active\_checks():FAIL
5892:20210918:182540.195 In process\_active\_checks() server:'zabbix.ait.cx' port:10051
5892:20210918:182540.196 End of process\_active\_checks()

None of the following resolved the problem:

- Uninstall Zabbix agent, delete the folder in Program Files, and reinstall the agent.
- Move the client computer to different network.
- Disable the Windows Firewall.
- Restart the agent daemon, the computer, and the Zabbix server.
- Delete the host from the server and let it be re-discovered.
- Increase agent timeout setting from 3 to 10 seconds.

[McThump]

Yes. As far as I can tell the hostname is identical on the server and agent. The agent autoregisters with hostname "GED131". This exact hostname appears in both the server and agent logs (see above). I even went into MySQL and dumped the hosts table. It also shows "GED131".

[McThump]

Each of the 2 hosts with the problem has a single IP address, but I hadn't thought about DNS. I'll check on that. Thanks.

[McThump]

I cleared the DNS cache and even changed DNS servers but the problem remains. Could a corrupted MySQL index cause something like this?

[McThump]

I never found the cause of this problem, but replacing the Zabbix agent with Zabbix agent 2 resolved it.

[cyber]

If your server says "cannot send list of active checks to ... host [] not found", then it is convinced, that it has no such host in its config. Is everything correct with hosts name etc.? Does it autoregister with exact same name? Capitalization etc?...

[cyber]

Multiple IP-s in host? Some mismatch in DNS?
TBH, I have never had such situation, so I'm just throwing things in the air here.....