Whoops! Hmm, something went wrong with a security certificate.
Recently Conor has been playing WoW and we came across an issue where the battle.net client displays this error rather than the latest Blizzard news.
I think it's safe to assume that Blizzard used an embedded web browser control for this section of the application, and most if not all of the recommended steps they provide (restarting battle.net, reinstalling, etc.) aren't going to fix the underlying problem, which has to do with SSL certificate validation going on in the requested page. What did actually fix this under OSX, was deleting the certificate in question from the OSX keychain. Read the rest of this for the how to.
When you open up the Keychain app, and locate the DigiCert High Assurance certificate expect to see something like this. Notice in this screen shot that the certificate is showing an error:
This certificate has an invalid user
Simply delete this certificate from your keychain/system.
Reload the browser and you should see the Blizzard news again.
If you still get the error, repeat the process and look for any other DigiCert certificates and delete those as well. I found 2 different ones in my keychain that had to be removed.