Ran into the error below yesterday after rebuilding an ESX 4.0 host as ESXi 4.1 and attempting to enable HA after the newly rebuild ESXi host was added back to the cluster. Cannot complete the configuration of the HA agent on the host. Other HA configuration error. Since error message was dissapointingly unspecific, I called VMware Support and asked for their guidance. According to my tech, they are actually getting a lot of calls on this error, as may who are doing in place upgrades to esxi from esx are running into this.