In setting up remote (push) notifications for iOS today, our manual server post to APNs was an error 400 Topic
The Apple Developer Portal stuff was fine, even though it said configurable. We were NOT using the old SSL method in iOS 9.
In our case, we had two App IDs, demo.baconinc (demo app), and another with was com.baconinc.bacon (production). In the Xcode project, it was still using demo, instead of the official App ID. Switched it, ran the app again, because you need to register again, and we got a good authorization token for the server to use.
So, if you have this error, take a look at your project App ID. That might be your topic error.