org.hibernate.InstantiationException: Cannot instantiate abstract class or interface:

After suffering this error message for a while the reason turned out to be almost logical:
In an @Inheritance of type Joined, there was an entry in the root table but no entry in any of the inheriting tables.

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Person {
…
}

@Entity
piblic class SpecialPerson extends Person {
. .
}

So in the table PERSON there was an entry with ID=100, bun no such entry in any inheriting table.