org.hibernate.PropertyAccessException: Could not set field value [STRING] value by reflection for String

Instantiate applikationUserPopupMessageId:

public class ApplikationUserPopupMessage {
  private ApplikationUserPopupMessageId applikationUserPopupMessageId = new ApplikationUserPopupMessageId();

That's known issue

Just put getters and setters for the according properties in your @Embeddable.

Does it help if you annotate your entity class with Access level to Field

@Entity(access = AccessType.FIELD)