I used to have a log in problem on a regular basis. Try this:
Close the game, Close the game completely (double tap Home button and swipe up on iPad, hit Square and swipe left on Android). Close ALL open apps.
Turn off your device.
Turn off all your internet equipment, Routers, Switches, Access Points, etc.
Wait 2 minutes.
Turn on your Gateway (modem). Wait for it to boot. Turn on all other internet equipment. Wait for all the lights to stop blinking. Turn on your device. Start the game.
I used to have to do that at least a couple of times a day. Major pain the the glass.