What does this error mean?
This error means an index of some type failed to flush to disk.
Why does this error occur?
This error should not actually occur unless a bug is introduced in the SecondaryIndexManager, triggering an assertion during index creation, truncation, deletion or flushing that logs this message.
How do you fix this error?
Since this should only occur if a bug is introduced, please file a ticket with DataStax Support to investigate.