(Disclaimer: I'm a developer for Nextpeer.)
This issue has to do with resource bundle mismatch. Note that 5 different resource bundles come with the framework, you should choose the one that matches your game (for example, if your game runs on iPhone and iPad in landscape mode, choose NPResources_iPad_iPhone_Landscape.bundle.
You can also contact us directly at support@nextpeer.com.
Cheers!