Q: What does APNS error actually mean?
A: It means the push notification certificate in your Apple Developer account, which enables your push notifications, has either expired or is faulty in some way.
Q: How often does this certificate expire?
A: It expires once a year, so expect to see this fairly frequently if you're a long-term client with quite a few apps. When the certificate expires, it completely disappears, as does your ability to send push notifications. The good news is Apple eventually enabled 30-day expiration notices. Now, 30 days before your certificate expires, they'll send you an email notification. When you receive these, just forward them right into email@example.com. No need to explain the situation: we know the drill!
Q: What would cause a certificate to become "faulty"?
A: Any break in the chain—if you transfer your app to another account, for instance—or error during the certificate creation process can cause a faulty certificate. Beyond that, sometimes there are just simple, inexplicable issues that arise with a certificate and require recreation.
Q: How can I avoid these errors?
A: By always forwarding us your 30-day expiration notices, and not diving into Developer on your own! Though it's tempting to log in and see if you can resolve something yourself, it's also potentially hazardous to your apps. Apple Developer is very complex for anyone, but particularly if you've never gone through the process of uploading an app. If any certificates are adjusted or removed, it makes it much more difficult to solve issues, since we can't account for changes made by people aside from our team members. On top of that, you don't have the necessary material to actually renew your certificates, so it's best to leave it in our hands.
Email into us at firstname.lastname@example.org with any additional questions!