Q I leave my Mac running all the time. When I got to it this morning, unusually the Mac App Store asked me to sign in. Then whenever I tried to run an app which had been purchased through the Store, it reported an error 173, told me that the app was damaged and had to be replaced, and quit. What is wrong and how can I fix it?
A For some reason – possibly a problem with the App Store itself – your purchased apps are failing their startup DRM checks with the Store. In essence, when you launch an app which you purchased from the Store, your Mac checks that the app’s signature is valid and licensed to your account. In your case, even though it should be valid, the Store is telling OS X that it is not.
Open the App Store pane in System Preferences, and check that the settings there are correct. If they are and the problem persists, you can try disconnecting from and reconnecting to the Store, and restarting, but you will probably need to contact Apple’s Store support for resolution. Otherwise, you will find that every purchased app will fail in the same way, forcing you to trash it and install it afresh from the Store. Thankfully that should not trash their settings, but it is exceedingly tedious to do.
This problem occurred quite generally from 2200 UTC on 11 November 2015, apparently because one of Apple’s security certificates (also involved in authorising the use of paid-for App Store apps) expired, according to the Guardian. If that was the cause, the only thing you can do is wait until Apple remembers to renew the certificate, and it propagates to your Mac.