Sometimes the game is freezing when I want to use Hawthorn. I last 3 battles in elemental class trial because of this so far.
Don’t use Hawthorn’s ability. This is a known bug. I am sorry for you.
I think it’s not Hawthorn’s ability that’s bugged (as we initially thought), rather that the [cast] button is turning green too soon when traits are creating gems and causing matches/cascades at the beginning of the turn. If you try to cast any spell before those cascades are fully resolved, your game will crash.
You should open a support ticket to request lost rewards for crashes during events with limited sigils.