IOS5 - Could not load NIB in bundle

I have an app in the appstore which works fine on any iOS < 5.0. But in iOS5 my app won't launch, the error I get is:

Terminating app due to uncaught exception 'NSInternalInconsistencyException', 
reason: 'Could not load NIB in bundle: 'NSBundle 
</var/mobile/Applicati开发者_如何转开发ons/****/***.app> (loaded)' with name 'MainWindow''

Does anyone know how to solve this error?

Update The app seems to work on the iPad 2 but not on the iPad 1 (now I am even more confused)

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The problem is probably that "MainWindow.nib" does not exist in the builded app. Either your xib is not named "MainWindow.xib" or your xib is not included into your project. YOu can check this by looking into the .app-directory on your phone or simulator.

I've got the same error yesterday and the problem was that I had a xib-file which was accidentally assigned a localization to. So maybe you have also added a localization to it then just remove it or provide a default one.