Check you VMKernel portgroup configuration (Management traffic) and also ESXi firewall configuration.

The address is not reachable via network between the hosts.

Try to ping other hosts from one host and repeat it on all hosts.

By default, the Management Network's default gateway address is used as the isolation address. Please check whether this is configured properly and has not accidentally been modified with e.g. configuring vMotion.

André

Seeing as the other hosts (assuming they are in the same subnet) can ping the isolation address (which is the default gateway for the management VMkernel as Andre points out), we can assume that the default gateway is an ICMP pingable device. Your best bet would be to check the specific host and see if you can ping the default gateway here. You can see what the default gateway is by running the command: esxcli network ip route ipv4 list, which would give you an output like this:

~ # esxcli network ip route ipv4 list

Network      Netmask        Gateway      Interface  Source

-----------  -------------  -----------  ---------  ------

default         0.0.0.0          192.168.1.1  vmk0       MANUAL

192.168.1.0  255.255.255.0  0.0.0.0      vmk0       MANUAL

192.168.2.0  255.255.255.0  0.0.0.0      vmk3       MANUAL

192.168.3.0  255.255.255.0  0.0.0.0      vmk1       MANUAL

~ #

The first line is your default gateway. That should be reachable for the hosts in a HA enabled cluster, as they ping this address to check if they have been isolated from the network.

Select the host giving this issue.
Go to Configuration > Networking

Locate the Management VMkernel Port group and click Properties

Select the Management Port Group and Edit > IP settings

Note the default gateway.

Right click the cluster > Edit settings

VMware HA > Advanced Options

Enter the following

das.isolationaddress under option

Enter default gateway IP under value

Click OK.

Right click the host > Reconfigure for HA. @

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Check your default gateway for the host.

Correct the gateway if you changed recently.