How to fix this OSM problem in X-Plane?

And if it REALLY bugs you you could make a little "fantasy airport" in that area with WED, placing an invisible helipad, for example and then covering the area with grass-taxiway and trees.

That's a great idea. You could also use marginal's overlay editor, and place a huge taxiway/apron over it, and then put a forest on the top of it. It will temporarily fill the hole in, and it's easier in OverLayEditor as you can see photo imagery from Bing