If You occured the problem in /var/log/zabbix-agent/zabbix_agentd.log
Message from <Zabbix Server IP> is missing header. Message ignored.
Downgrade Zabbix Agent from 4.x to your server version.

# apt-get remove zabbix-agent
Download appropriate version from http://repo.zabbix.com/zabbix/3.0/debian/pool/main/z/zabbix/
# wget http://repo.zabbix.com/zabbix/3.0/debian/pool/main/z/zabbix/zabbix-agent_3.0.28-1%2Bbuster_amd64.deb

install it
# dpkg -i zabbix-agent_3.0.28-1+buster_amd64.deb

lock if for future upgrades
# apt-mark hold zabbix-agent

unmask
# systemctl unmask zabbix-agent

enable and start
# systemctl enable zabbix-agent
# systemctl start zabbix-agent