What does this error mean?

This error means that there are either redundant SSTables on disk that should have been deleted, or that there are new SSTables that are not usable.

Why does this error occur?

This error occurs when a process fails while Cassandra is completing or aborting the transactions associated with an operation (e.g. upgrades stables).

How do you fix this error?

In most cases, you can safely avoid the error, and attempt the operation again. If the error continues, and you cannot complete the operation, check the log entry after the error that is preceded by 

Full interruption stack trace

This will provide more details as to the root cause. If you need further assistance, you can provide this information to DataStax Support when filing a support ticket