What does this error mean?

This means an error occurred when calculating either the sampleReadLatencyNanos or additionalWriteLatencyNanos in the ColumnFamilyStore.updateSpeculativeThreshold() method.

Why does this error occur?

When updating the speculative retry options via CQL, it is possible for an exception of some type to occur during the moments that Cassandra is performing calculation. This generally does not occur, but a catch all for throwable errors exists in the try-catch block for ColumnFamilyStore.updateSpeculativeThreshold(), which generates the message.

How do you fix this error?

Because this is a catch all error message, it is important to look at the stack trace that accompanies the error message to determine the cause. You may need to simply execute the CQL statement again to update the speculative retry.