The only thing I know of that can cause this error is if you are forcing resource fallback to use something other than English when keys are missing in the non English resource files.
ie you need to have this in web.config:
<globalization culture="auto:en-US" uiCulture="auto:en-US"...
because English is the only language guaranteed to not have missing keys.
You should not change this setting, you can still force another language by other settings but this <globalization setting should never be changed.