What does this error mean?

This error means a ResourceWatcher thread had an exception thrown while monitoring a resource.

Why does this error occur?

In theory, this ResourceWatcher class is meant to be somewhat generic, and this error would correspond to an error in the execution of the generic runnable passed to that class. In practice, this appears to be only used by the PropertyFileSnitch (and GossipingPropertyFileSnitch by transitivity. So in practice, one would get this error if the cassandra-topology.properties file is modified on a node after startup (that file is live-reloaded if modified) in such a way that the resulting file is invalid (typically, a syntax error or a bad configuration).

How do you fix this error?

Check the stack trace that follows the error message for more details on what the specific cause is. Make the adjustments in the file and wait a few seconds for Cassandra to re-load the file.