2022-01-27, 15:00 UTC: We are aware of an issue that can cause the game to freeze when starting a battle in the Challenge Festival I event. This will be fixed in our upcoming Version 55.
In the meantime, the issue can be prevented by following these steps:
After tapping “Fight” to begin the stage, you will be shown a dialog explaining the special rules for that stage.
Tap the blue arrow on the bottom right to dismiss the dialog and continue to your battle. Tapping anywhere else outside the dialog will dismiss the text and cause the game app to freeze. Make sure you tap the blue arrow.
We apologize for the inconvenience. Thank you for your understanding!