What does this error mean?

This error means that a Cassandra node was not able to connect to another node.

Why does this error occur?

This error is caused when a node attempts to reconnect to another node and encounters an UnknownHostException. Typically, this is for situations like EC2 where a node will have a public address and a private address. Cassandra initially connects on the public address, discovers the private address, and then reconnects on the private address.

How do you fix this error?

Since the exception encountered is an UnknownHostException, check the DNS settings on both nodes as well as the broadcast_address and listen_address settings on the node you are attempting to connect to. Any stack trace that follows the error message should be able to narrow down the potential causes. DataStax Support can assist if needed.