Challenge
Backups fail as a result of a vSAN cluster node being offline.
Cause
If the vSAN has fewer than 4 nodes and one of the nodes is offline, either for schedule maintenance or unexpectedly, snapshots cannot be taken of VMs on the vSAN cluster.
Solution
VMware recommends having at least 4 nodes for extra fault tolerance in the event that a node would unexpectedly go down.
More Information
This can occur with clusters of more than 3 nodes if enough nodes are offline to take the cluster down to 2 or fewer online nodes.
For more details, please see: https://kb.vmware.com/s/article/2106708
For more details, please see: https://kb.vmware.com/s/article/2106708