The vSphere HA agent is not reachable from vCenter Server

Today I placed a host in a cluster into maintenance mode and the virtual machines migrated to other hosts (DRS took care of this) without any issues.  However, a few minutes after it had moved the VMs I received an alert from a different host in the cluster:

The vSphere HA agent is not reachable from vCenter Server

This was easily resolved by right-clicking on the host and selecting the ‘Reconfigure for vSphere HA’ option.  Within moments the host had reconfigured itself for HA and the alert was cleared.