Steps to reproduce:

1. Update Zabbix server to version 3.0.19
2. Update Zabbix proxy to version 3.0.19
3. Restart Zabbix proxy or run zabbix_proxy -R config_cache_reload

Result:
Zabbix versions check:

root@zserver:~# zabbix_server -V
zabbix_server (Zabbix) 3.0.19
Revision 82159 25 June 2018, compilation time: Jun 25 2018 08:17:54

Copyright (C) 2018 Zabbix SIA
License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it according to
the license. There is NO WARRANTY, to the extent permitted by law.

root@zproxy:~# zabbix_proxy -V
zabbix_proxy (Zabbix) 3.0.19
Revision 82159 25 June 2018, compilation time: Jun 25 2018 08:15:54

Copyright (C) 2018 Zabbix SIA
License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it according to
the license. There is NO WARRANTY, to the extent permitted by law.

From /var/log/zabbix/zabbix_proxy.log:

30716:20180711:060236.878 received configuration data from server at "xxx.xxx.xxx.xxx", datalen 80591
30716:20180711:060236.879 failed to update local proxy configuration copy: unexpected field "hosts.available"

The problem is consistent through several proxies all of which has been updated to 3.0.19 version together with the server.

Expected:
Proxy should be able to update its local copy of proxy configuration.