What does this ERROR message mean?
This means that an exception happened in a particular phase of server shutdown after normal shutdown. Post-shutdown hooks are routines that are run after normal shutdown of the service.
OSS Cassandra doesn't currently have any post-shutdown hooks, so this message is impossible to see for that piece of software. This is a feature that allows for routines to run after normal shutdown of the Cassandra service, but Cassandra isn't currently using it.
DataStax Enterprise does use a post-shutdown hook, but core exceptions are swallowed in the post-shutdown hook itself (before they can get to the post-shutdown hook manager) and only a small set of statements involving logging and a java.util.concurrent.CountDownLatch could possibly throw exceptions here that are then printed out by the post-shutdown hook manager.
Why does this ERROR occur?
Either a logging routine or java.util.concurrent.CountDownLatch operation further threw an exception.
How do you fix this ERROR?
You'd need to look at the specific exception that happened in the post-shutdown hook. Depending on the specifics there, you may need to do something about your logging config/environment. If the exception is related to CountDownLatch, please file a bug report with DataStax.