Unfortunately, it's hard to pinpoint this type of error. ("Buffer creation failed. Internal error.") Personally, I regard anything marked as "internal error" as a bug of AIR/Stage3D, because if your code would do something wrong (e.g. let the game run out of memory or make an invalid stage3D call), then they'd be able to return a more meaningful error.
I'd say, keep it under surveillance and check how often it really occurs, but don't spend too much time on it as long as it's just a rare incident.