Game gets stuck constantly in NOTB

I am usually doing the trooper grind while farming NOTB with Alex Ed (Valor) and two random troopers to level up (often unsharded and w/o talismans). Since the last couple days, however, the game gets stuck regularly when entering the 4th wave (W + A COTD, 2 WHH). Haven't found a pattern yet, since the game runs on auto and I do not pay attention too much during the grind. However, it seems that on all occasions when the game got stuck, both troopers had died and Alex Ed was the lone character standing.

I think I remember reports of similar things happening in the Wicker Man Event Dungeon. So maybe it is a more general problem.