Can confirm Cody's observations.

We startet getting these error messages es masse when we migrated from WildFly 10.1.0 to WildFly 15.0.1.

We created our own custom WildFly (using Hibernate's Maven plugin), so now we're running WildFly 15.0.1 with Hibernate 5.3.9. Still, those error messages appear.

Solution is to disable JDBC batching for the time being as we do not know if the errors might lead to data loss. But this is a noticeable performance degradation...