Thanks rommulus. I waited for a half hour without Corona stopping. How long time did you wait? I also tried lowering the AO samples, but didn't see any difference. I haven't seen this in older versions (used Corona since a5), but maybe I've just been lucky.
This scene is made in Corona (can't remember exact version) and has been updated along with Coronas progress. But this is the first time I've run in to such a severe bug. I really hope it's going to be fixed soon, so I can benefit from those nice new enhancements, also when I'm using AO.