Recently after restarting a vSphere host I was greated with the message.

VMware_HA_Agent_Error

For me the issues was simply resolved by right clicking on the host and selecting “Reconfigure for VMware HA”

VMware_HA_Agent_Error_2

If that does not work then check the network configuration of the Service Console on the effected Host and make sure the Host is able to ping its default gateway. Also try restarting the Service Console first