So I have been testing out ZGC with a Cassandra 4.1.6 cluster, and been seeing the same type of errors. ZGC is an experimental feature in java 11, and I strongly believe the error you are seeing is due to it being an experimental feature. Upgrading to Cassandra 5.0.x allows you to run ZGC in java 17 (ZGC was promoted to a Feature in Java 15). I think running ZGC at least for production is not recommended. Not sure if this answers your question, but at least it is an indicator why you are seeing this error.