User blog comment:Semi-Squishy 3000/How to Bypass Crashing on iOS/@comment-85.158.137.195-20150724054754

The server is unstable. The best we can do is sit back and wait, since gumi is working on it. They didn't expect the connection issues, so they weren't prepared for it. It happens to all of us. The above mentioned actions tend to work.

What you also can try is using an app to force close the background process for BF once, and have that boot back up when you boot the game.