@Marek1 @vsgoncalo The info provided previously, regards deprecation of Transport Client authentication/authorization. Port 9300 will be still in use for transport traffic.
This affects your deployment too. Once you disable demo certificates and you’ll use custom TLS certificates, you’ll need to add all your node’s certificates to plugins.security.nodes_dn in opensearch.yml file.
The error will appear if the node with TLS certificate, that is not in that list, will try to join the cluster.