There is no "virtual nand", it's just files stored directly on the HDD. The only way it could be full is if the HDD was also full.
It's probably some other bug like a sharing issue or invalid characters in the filename, and the game doesn't have proper error handling so it just assumes any error means insufficient space.
It's probably some other bug like a sharing issue or invalid characters in the filename, and the game doesn't have proper error handling so it just assumes any error means insufficient space.