I know what I did wrong:   I shut down a host in my cluster without first putting it in maintenance mode.  All other hosts in the cluster complained with the same warnng:

vSphere HA agent on this host cannot reach some of the management network addresses

The host that I shut down was long gone ; I couldn't add it back to the cluster.   I did as suggsted above:

I stopped HA for the cluster.

I started HA for the cluster.

My cluster configuration settings were maintained; there was no need for me to configure anything else.

All of the hosts in the cluster cleared the warning message.