This minor version doesn't have any upgrade notes.
Zabbix server 6.4.12 and Zabbix proxy 6.4.12 are not compatible with other versions of proxy/server. If either server or proxy is 6.4.12, then both server and proxy must be 6.4.12.
This issue will be fixed in the next release. However, while the next release will be compatible with 6.4.11 server/proxy (or sooner); it will still not be compatible with 6.4.12 server/proxy.
To prevent critical security vulnerabilities, the minimum Windows version for Zabbix agent 2 has been raised to Windows 10/Server 2016. See note under Supported platforms for more information.