Foundry VTT over NGINX Reverse proxy

Hi everyone

First a quick overview on my local network:

AVM Fritzbox (ISP Router) --(Exposed Host)--> Ubiquiti Dreammachine Pro --(Port Forwarding)--> Nginx Reverse Proxy --(Proxy Host)--> My PC

I have a Foundy VTT running on my PC, which runs on port 40000. On the DMP, there is a port forwarding configured for 80,443.

"Outsiders" can reach the server per se, but getting nothing of a content, only the base website framework (like the frame of a painting, but not the painting itself). The reverse proxy seems to be working, but I guess, I have to configure something more. I already tried to configure a Stream on nginx for port 40000, but thats not working, either. You can see both displays here

Can anyone provide any help in this case?