App store submission problem with MPViewController

I'm getting the following email when submitting an app to the iTunes App Store:

Dear developer, We have discovered one or more issues with your recent delivery for "~~~~" To process your delivery, the following issues must be corrected:

Non-public API usage:

The app contains or inherits from non-public classes in ProductName: MPViewController

If method names in your source code match the private Apple APIs listed above, altering your method names will help prevent this app from being flagged in future submissions. In addition, note that one or more of the above APIs may be located in a static library that was included with your app. If so, they must be removed.

This appears to be related to the problem from this post: http://forum.unity3d.com/threads/submission-error-with-mpmovieplayercontentpreloaddidfinishnotification.267261/

However, none of the solutions listed worked for me.

I'm using Unity 4.6.5 Pro, and I've tried both the latest version of XCode and version 6.0.1

Thanks in advance for your help!