Hi,
If a reboot of the host or management agents have fixed it, it was probably an issue with the host itself not being able to ping the HA isolation address.
This, by default, is the default gateway on the host. If a reboot/restart hasn't fixed the issue:
1) Double check what your configured default gateway is on the host.
2) Login to the host esxcli and try and ping the DG manually.
3) It could be an HA issue. You could try disabling HA on the cluster and re-enabling it to force it to reconfigure.