I have investigated this problem a little further.
I exported using the "Product > Export Localizations... > MyApp Workspace..." option, then imported the xcloc file to the project without any touch. I got the same error.
I exported using the "Product > Export Localizations... > MyApp..." option, then imported the xcloc file to the project without any touch. This time I didn't get the error above, instead, I got a warning window, full of "Empty Translations" and "Over-Localized Do not Translate" warnings.
Strangely, these warning are mostly about the "note" lines of the translations, that are written between /* and */ tags. These lines are coming from comment strings in the example code below:
NSLocalizedString("Hello!", comment: "Greeting in the welcome screen")
You aren't supposed to translate "Greeting in the welcome screen" here, but Xcode gives "Empty Translations" warnings about them. I wonder why.