Unfortunately, a caching issue with the Mac App Store meant that some users had to restart their systems and re-authenticate with the Mac App Store to clear a system cache of some outdated certificate information. We are addressing this caching issue in an upcoming OS X update.

Also, some apps are running receipt validation code using very old versions of OpenSSL that don’t support SHA-2. We addressed this by replacing the new SHA-2 certificate with a new SHA-1 certificate last Thursday night.

Apple responds to the MAS certificate issues. pic.twitter.com/WseQuAIaWN

— Dirt Don (@dirtdon) November 18, 2015