EDIT (18 Oct, 08:20 AM UTC): We have released an update for both Android and iOS that should fix this issue. Please update your game!
There is currently a known issue with the new update version 24, where the game crashes with a Connection Error message when playing Season 2 stages. This seems to occur only when the player has attempted to play both Normal and Hard difficulties of any specific Season 2 stage.
As a temporary solution, please try force closing the app when attempting to switch between the difficulties.
We hope to release a new update to fix this issue as soon as possible. Our apologies for any inconvenience caused by this issue!