This is the same problem as JBPM-2474.
The problem is in HibernateIdMatcher.
if (sessionFactory!=null)

{ ClassMetadata classMetadata = sessionFactory.getClassMetadata(value.getClass()); matches = ( (classMetadata!=null) && (classMetadata.getIdentifierType().getClass()==getIdType()) ); }

value.getClass() is returning the Javassist proxy class that is not found.
It needs to check for "value.getClass().getSuperClass()" if value instance of HibernateProxy.