Zabbix Proxy cannot send data to Zabbix server, get an empty string

Because I have the responsibility of server maintenance in my work, in order to be able to find problems in the server in time, I plan to use zabbix for monitoring. However, there are only a few servers with fixed public IP addresses, so the zabbix proxy mode is adopted to monitor other servers without public IP through the proxy. It was originally a very simple matter, and I have successfully installed and deployed zabbix proxy before, but I did not expect to encounter the following problems after the deployment and operation:

began to think that the Zabbix proxy and Zabbix server between the network caused, after the inspection found that the network is not a problem.
It's strange to check the configuration and not find the problem.

Finally in a Zabbix group raised this question, some people say that may be Zabbix proxy version and Zabbix Server version inconsistency caused, I quickly checked the version, Zabbix Proxy is 4.0, and Zabbix server is 3.4.
Proxy is higher than server version.

After the proxy version has been reduced, the problem has finally been solved, it seems that after the deployment of proxy, we must pay attention to the version.

Zabbix Proxy cannot send data to Zabbix server, get an empty string