Hmm, yeah, not sure what I was smoking.
Anyway, it was confusing to me that your structure is *below* the ground plane not above it. Since the copper forms a nearly perfect conductor, there is no need for an air box above it. You can delete all that air and instead put it below the microstrip, where there is only a little bit of air now and more is needed. That might solve the problem.
You only need an airbox where there will be fields. Above the copper plane there will be no fields; below the strip there will be significant fields.