What does this error mean?

A throwable error of some type was caught in the constructor for the ExternalArchiver class when attempting to archive .cq4 files for the BinLog. 

Why does this error occur?

This is a catch-all error message. Any throwable error could trigger this. There is not a specific cause that can be called out. In this case, you shouldn't need to do anything. Cassandra will attempt to retry the archival until it reaches the maximum number of attempts. 

How do you fix this error?

Please look at any available stack trace associated with this error to determine a root cause in order to plan a fix for it. If needed, Datastax Support can help.