To quickly fetch the causing exception without the need to re-code or rebuild, set a breakpoint on
org.hibernate.ejb.TransactionImpl.setRollbackOnly() // Hibernate < 4.3, or
org.hibernate.jpa.internal.TransactionImpl() // as of Hibernate 4.3
and go up in the stack, usually to some Interceptor. There you can read the causing exception from some catch block.