this is a recurring but very rare occurence on express

it is not really a black screen
it is that the news of your attack for some reason does not publish across from your turn options like market, search, spy center etc but below it and you can not scroll up

the reason i say it is not a black screen is because u can see the tiny gray arrow on the far right of the screen

so u have to go to the public news to see if ur attack was successful

i am experiencing this bug right now on express