It was both. Turned out that in a CocoaPod imported lib there was a category on UIAlertController that returned in -supportedInterfaceOrientations UIInterfaceOrienationPortrait instead of UIInterfaceOrienationMaskPortrait.

Thanks a lot!

The UIImage is just a plain [UIImage imageWithData:...] generated in a background thread, the UI update is dispatched to the main thread. I couldn't reproduce it yet. The code is run on every app execution, the probability to have this crash seems to be 1:5000.