Just pasting links to the other topics here 'cause they're kinda related.
viewtopic.php?f=9&t=3999
viewtopic.php?f=9&t=4025

@Rosss. You'll need to throw a bunch of Debug.Logs or try-catches in there to track down where the snag is, since the stacktraces from Unity's IL2CPP (for iOS 64-bit) doesn't give you line numbers.