Won't let me log in after update

OK, updated app, logged in and went to the new area. Upgraded guns, played two rounds. Went to log in again and I get a message telling me that I need to go to the App Store and upgrade the app. Only option is to click "open app store." When I do, it shows that the game has been upgraded. When I go back, I get the same thing. Caught in a loop. IPhone 6s Plus 128GB.