Same issue on 2 separate clusters (both have 2x Server 2016 nodes) after cluster aware update (which only done first node!) As CUA failed the second node never updated & that is why cluster still works!
Both have failed cluster:
Node '' failed to form a cluster. This was because the witness was not accessible. Please ensure that the witness resource is online and available.
Cluster node '' failed to join the cluster because it could not communicate over the network with any other node in the cluster. Verify network connectivity and configuration of any network firewalls.
There is nothing wrong with witness disk, it is accessible fine, shows in Disk Management as Reserved (exactly as it should)
There are known issues with this update, but none states that it would kill Cluster!
In process of testing on clusterA I removed the Down node & tried to add it, cannot do it!
On clusterB, it is now one node working cluster and the second node cannot join. The cluster will work from either node (if I Shutdown both nodes & power them in different order)
I am able to ping the host names and FQDN's of both nodes and the DNS server from all servers