That's good to know. I already started looking into it and also played a bit after the first boss with kitsune forms but the game did not crashed for me once till now.
The error seems to be able to not load an actor (Partymembers) image. But i have no idea why it would happen only there. After all, all the party members have images. Did you use the transformation skill in battles there? That is the only skill that can change Nana's battle image.
I will try again while copying your party layout.
Sorry for the incomvinience!