If you are using group policy (and why wouldn’t you) to configure your WEM agents, pay close attention to leading and/or trailing spaces. This has happened twice to me with agent versions 4.2 and 4.3.
The Connection Broker Name and Site Name fields in the Workspace Environment management\Agent Host Configuration administrative template must NOT contain any leading or trailing spaces. If you have any, then the agent cannot communicate with your WEM Broker server nor can it load the correct Configuration Site.
Thanks for reading,