I investigated the error some more and I'm 100% sure it's an area-specific error related to the Nautilus Pet (not related to Naija herself).
The area:

I entered the problem area again without changing into energy form even once. Then the error popped up AGAIN. I had the Nautilus Pet out just like last time. When I hid the pet, the error disappeared.

What I think is that the Nautilus Pet and some stuff in the area don't like eachother. Truth is I barely use that pet in the first place since it hardly helps, but since it's the only one I have I keep it out for just that little extra. But guess what - it's animation clashes with the animations in this area.
I can't check other pets, sadly. I have no other ones at the moment and I lost my file where I have like every dungeon and sideboss completed except for The Body and the things after that.
I'll report back if it shows up in other areas/circumstances.
