1. We have monitoring many systems with equivalent settings. But only 3 server have issue like described.
2. I had no this issue with Client version 6.4.7 or early

Steps to reproduce:

1. Set in zabbix_agent2.conf :
   LogFile=c:\zabbix_agent2.log
   Server=zabbix-server.dns
   ServerActive=zabbix-server.dns

Hostname=
HostnameItem=system.hostname
HostMetadata=
HostMetadataItem=system.uname.

Result:
In log file: 
cannot get host metadata: Class does not exist.
cannot get host metadata: Class does not exist.
cannot get host metadata: Class does not exist.
and after some times 
cannot get host metadata: Not enough memory resources are available to process this command.
Agent restart can't resolve this issue.