I think I fixed it. The courtyard is now really shown. From my point of view the issue was that there were far too many relations and some building outlines (or parts) were made of multiple ways which were combined again in a relation. I would not do that but have each building part as one outline. JOSM still complains about overlapping building, that is due to the building parts with the different roof shapes. That might be something one day someone can look into. Though I believe there are other more important issues than that. Hope it helps and satisfies everyone.