ObjectDB uses subclasses of the standard JPA exception classes in order to add functionality (e.g. by overriding the printStackTrace method, additional information, such as the ObjectDB version that is in use, is printed).
Particularly, com.objectdb.error.jpa._EntityNotFoundException is a subclass of javax.persistence.EntityNotFoundException.
Which ObjectDB version do you use?
If _EntityNotFoundException is not recognized as a subclass of EntityNotFoundException, maybe this is because of loading EntityNotFoundException more than once using different class loaders. Please check that the JPA classes are available to your application only once.
ObjectDB - Fast Object Database for Java (JPA/JDO)