Connection failed for host: failed to install/upgrade vCenter Agent service.
There is a conflict with an upgrade or installation of the host agent.
For ESX(i) 4.0, refer to this VMware KB article. For information on resolution steps, review the VMware KB article Diagnosing the vSphere Client when it fails to connect to an ESX/ESXi host or vCenter Server (1003870).
Refer to the VMware Online Documentation for more information.