If you try to download an app or update (ours, or from any other developer) from the Mac App Store and get this message...

App Store Error: Failed to verify the preflight file. It is not signed by Apple.

...the problem is (most likely) caused by a setting in the Keychain Access app. Make sure the pop-ups for OCSP and CRL in Keychain Access > Prefs > Certificates are both set to "Best Attempt." This should fix the problem; more details are in this blog post.