UnknownEntityTypeException: Unable to locate persister after Migration to 5.2

After Migration from 4.1.19 to 5.2.17 we got

org.hibernate.UnknownEntityTypeException: Unable to locate persister:

when calling

Session.get(Class clazz, Serializable id)

We don’t use Hibernate/JPA-Annotations. We still use legacy HBM mapping-files.

Here’s what the Initialization-Method for the SessionFactory looks like:

private synchronized SessionFactory getSessionFactory() {
	private SessionFactory sessionFactory;
	try {
             // Create configuration 
	     Configuration configuration = new Configuration();

	     // add custom-Hibernate-Interceptor
  	     configuration.setInterceptor(new myInterceptor());
	     // Build a Service-Registry
	     aServiceRegistry = new StandardServiceRegistryBuilder(bootstrapRegistry)

             // Build the MetadataSources
	     MetadataSources aSources = new MetadataSources(aServiceRegistry);
   	     // Build the Metadata
	     Metadata metadata = aSources.getMetadataBuilder().build();				
	     // Build the SessionFactory
	     sessionFactory = metadata.getSessionFactoryBuilder().build();
       catch (Throwable ex) {
       return sessionFactory;

Try to replicate it like this so we can investigate it further.