[eridius]

There seems to be a bug in the system where sometimes it reports an app as damaged when it's perfectly capable of downloading a new receipt (this is why a reboot works, because the bug seems to only crop up after some period of usage). When I bought a new iMac and transferred all my stuff from my old computer, I had this exact same issue, all of my MAS apps were reported as damaged. After reinstalling one, I tried rebooting, and that worked; got a password prompt for the next MAS app, and the rest just worked fine at that point.

Hopefully they're aware of what's causing this bug and will fix it in an OS update.

Of course, when the certificate is actually expired, and they haven't replaced it yet, then the "damaged" dialog is reasonable, because the system cannot get a valid replacement receipt. But once they replaced it, OS X should have just started working again. In fact, I'm going to go file a radar right now.