EDIT (9 Apr): We have now rolled out the new update 28.1. for all players, which should fix this issue. Please update your game!

There is currently a known issue in version 28.0 where the Challenge Event view is stuck when trying to exit is by tapping the red X button. To resolve this, please restart your game. 
This issue will be fixed in the new update version 28.1 which we have started rolling out for both iOS and Android.

Our apologies for any inconvenience caused by this issue!