The * .storyboardc file was not found.
I think this is a mistake. I solved this problem when I moved Main.storyboard (NSExtensionMainStoryboard defined in info.plist) into a subdirectory of the extension target. This means that you cannot localize main.storyboard because it will be moved to the base directory or a specific language directory.
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I ran into this problem and found the following to work without sacrificing localization.
1) Copy your Main.storyboard from en.lproj to the root of your extension
2) Drag the copied file into Xcode among the extension files and make sure you don't need to "Copy items as needed"
3) Archive and check
This means that it passed the validator check. I have a developer support ticket at Apple and they are aware of the problem.
UPDATE 10/15/2014: Apple has resolved this issue. No workaround required anymore
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