What does this error mean?

This error means that Cassandra failed to COMMIT or ABORT a transaction.

Why does this error occur?

During compactions, Cassandra tracks SSTable files using a transaction mechanism. If Cassandra fails to COMMIT or ABORT a transaction for some reason (e.g. transaction already completed, committed or aborted ...) the is logged along with the exception that caused the failure.

How do you fix this error?

There is no specific fix for this error. Further investigation will be needed. A support ticket should be filed with DataStax Support.