This article explains the network - server cluster partition check in the vSAN health service and provides details on why it reported an issue.

VMware vSAN 7.0.x

Q: What does the network - server cluster partition check do?

When the remote vSAN datastore is mounted in the client cluster, it will check if all hosts in client cluster connected to all hosts in server cluster, e.g., which server hosts are available to each client host during network partition. For hosts in one cluster that are not connected to any hosts in the peer cluster, their server partition status remains unknown. Hence those hosts will be shown in the unknown partition group.

Q: What does it mean if it shows an error state?

Normally all hosts in the client cluster (e.g., client hosts) mounting remote vSAN datastore should be able to communicate with all hosts in the server cluster (for example: server hosts) exporting the same vSAN datastore. In the case of network partition, however, client hosts may only see a subset of server hosts. It is unlikely but possible that different client hosts see different server hosts. Object availability will be impacted for as long as the partition lasts. 

Q: How to fix the error state?

Checking the network partition health status in server cluster and make sure there is no partition, secondly, checking if all hosts in client cluster can communicate with each of host in server cluster through vSAN vmknic.