I've encountered this issue 3 times today. When playing the Heroic Demonic Invasion level, the game will just suddenly get stuck (usually after 6-7 kills). It doesn't freeze, but no new enemy units will spawn, hitting the "End Turn" button repeatedly does nothing, my heroes' turns don't refresh and the game just stays in that state until my battery runs out, regardless of whether manual or auto mode is used, right from the beginning.
The only way to get out of it is to retreat and waste that 10 energy, and hope that the next event fight won't have the same issue.
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I've tried force-quitting, restarting the app and resuming the battle, restarting the phone, including taking out the battery and putting the battery back), clearing the phone cache and other residual data on the phone, but the same issue will occur, regardless of whether I play auto or manual. This issue seems to have only started when the event ladder started.... So how now? Just give up on the event ladder?