Zabbix Server - 2.0.2
Zabbix Proxy - 2.0.2 (mode: Active)
Zabbix agent 1.8.2 monitored by proxy.
After upgrade agent to 2.0.2 I have the following situation:

• No data from zabbix_agentd collected by server(via proxy)
• In frontend Availabilty of agent is green
• In bash : zabbix_get successfully get data from zabbix_agentd
• Data from not upgraded agents still collecting

I updated about 20 hosts one by one, very interesting thing:
After upgrade of agents I have situation above. Then I perform "zabbix_proxy restart" and I begin to see data from only some of upgraded zabbix_agents(2.0.2) but not all.
To see more I perform one more zabbix_proxy restart.

Database on Proxy is fresh (was built from 2.0.2 scripts without upgrade)
Database on server was upgraded from 1.8.10