It's after I unpause the game the first time after entering into the game. Oh and in the installation instructions of course.
EDIT: I feel like I should clarify. I get the error message when I first unpause, but the error message itself doesn't refer to the file. The message is the missing files one you refer to in the instructions, and the only reason I can think of for getting it is the lack of the file I mentioned, since it's the only thing different from the instructions.
Stupid question, but where are we supposed to get the "_main_bulk_customized_animations.json"? It's not in the package that I can see. I tried even deleting all other mods and following the instructions exactly. Dragged the mod_data folder into the Mods folder, but I get the error you warned against. I assume because the "_main_bulk_customized_animations.json" isn't present.