What does this ERROR message mean?

This error means some exception was thrown during a repair process. 

Why does this ERROR occur?

This error message is logged when an error is thrown and then passed to the notifyError method in the RepairRunnable class. The error provided will be a known error (meaning that it is not a SomeRepairFailedException (A repair failure that Cassandra cannot access the actual exception). The error that was thrown will be provided immediately after the logged message above.

How do you fix this ERROR?

The notifyError method is a catch-all method that can have many different errors passed to it. For instance, the run() method for the thread has a try-catch block that catches any general exception while PreviewRepair and RepairSessionCallback methods have internal methods that catch generic exceptions. Each of these will call notifyError to log the error. Since the errors could be of virtually any type, you will need to look at the specific error, message, and associated stack trace to determine the cause. DataStax Support can assist if you are unable to determine the actual cause.