Numerous Apple customers found themselves unable to download and run Mac apps from the App Store on Wednesday night. It appears the service outage culprit is Apple itself. More specifically, the culprit was an anti-piracy security certificate that Apple issued back in 2010 but which expired last night just before 10 PM GMT. Once developer Paul Haddad spotted the error, however, Apple quickly issued a new certificate to get the store back up and running.
Source: The Register