Getting black screen in gym battles

All of a sudden a problem has appeared when I do gym battles. When I enter a gym battle, the screen goes black. I can see all the written stuff (e.g, user names, pokemon names, the "GO" starting message) and the colored bars showing damage and energy, but not the pokemons or the battlefield (it's completely black). I can be attacked by the defender and take damage, and If I tap the screen, I can do damage back. I can see everything before going into battle (e.g., the defending lineup) and the rest of the game works fine. Anyone know what's going on and how to fix it? [I have an android phone.]