I have experienced the exact same symptom yesterday and today, attempting to download the newly released 6.0.1 update.
This problem persisted after performing the following steps.
Trashed the 6.0 Keynote.app file.
Signed in and out of my app store account.
Clicked the checkbox agreeing to Apple's terms and conditions and saved.
Reset all warnings for buying and downloading (in the user account info screen).
Trashed the com.apple.appstore cache folder (with and without cold booting after trashing).