Well, sorry if my first comment was a bit confused. But I played around a bit with ComfyUI on my own and now I understand better the issue.
I was able to run smoothly the workflow, after installing all the missing nodes thanks to ComfyUI manager, and by draging myself the 3 png workflow on the web interface of ComfyUI.
But the game still has the failed request 400, so for some reason it is unable to share the workflow to ComfyUI and to send it requests