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.
Specfically the error has to do with the fact that ESX has both a Service Console and VMkernel port used for host management, and ESXi only has a VMkernel port. To correct the issue and allow ESXi and ESX to place nice in the same cluster you must add the following.
First under VMkernel Properties make sure that both VMotion and Management traffic have a check mark to indicate that they are enabled.
Second right click on your cluster, and select VMware HA. Then click on Advanced Options and add the following 3 entries.
das.allowNetwork1 = Service Console
das.allowNetwork2 = VMkernel
das. allowVmotionNetworks = true
See screenshot below.
UPDATE: Note that by default the original name of the management port group is, Management Network. You must change this to VMkernel for this to work.