App Store can't verify purchased applications This morning, when launching an application I purchased from the App store some time ago, I was greeted with a sign-in box telling me that Apple needed to verify the application.  I signed in using my iTunes password (which I've since double checked, and I know I entered accurately), and was greeted with the following message in reply:

Your device or computer could not be verified. Contact support for assistance.

Some poking about online shows one thread from a couple years ago where someone says he called Apple and they told him to delete "the plist", but that's all the information given.  Deleting ~/Library/Preferences/com.apple.AppStore.plist did nothing to change the issue, however.

A couple of these applications are required for work.  Any ideas on what the issue is or how to fix it?
Short of calling Apple Support (or web chat or twitter @AppleSupport) you could check a few things:


date and time are correct and set automatically
sign out and in to the App store with your Apple ID
delete the app in Launchpad and download it again (make a backup in case the app has been pulled from the store)
make a new user account - delete the app and re-install it from the new user
restart the OS