Could you be using the Knuckles as a weapon? Someone identified that there is a bug with them and occasionally they cause battles to crash.
After your tip, I went and tested it and it has been confirmed that using “Fast Atk” with “Knuckles” equipped will cause the game to crash.
Other weapons have been tested differently depending on their effects, and no serious issues have been found yet, except for the “Dagger” and “Kite Shield” effects which do not trigger stuns and sound effects on enemies other than the target.