What does this error mean?
This error means an unsupported transport message was encountered from a legacy protocol version.
Why does this error occur?
This error occurs when a Cassandra node is running an older version than the rest of the cluster and transmits a message that is invalid.
How do you fix this error?
This error should likely only occur during upgrades and can be ignored. Once the cluster upgrade is completed, all nodes will be running a non legacy protocol version.