"Loading Data" screen after switching to a different network (need help with diagnosis)

Hi all,

I preconfigured a dockerized homeassistant installation at home for a friend of mine just like I have mine at home running.
The setup is quite simple: I have a VPS that runs a wireguard server. An nginx reverse proxy handles all my requests from outside via different domains, tunnels them through my wireguard network and points them to my wireguard clients (the raspberries in my home network, one for me, the other one that I configured for my friend)

The good thing is, I never have to configure any static IP address since it only runs through the wireguard tunnel and once the handshake is done I can access my Homeassistant from anywhere.

However, when I took the Pi and plugged it in on my friends network, I was only able to access the local page but from my domain I only received a “loading data” page. (The log on my server however returend Statuscode 200)

My question now is as follow: Do you have any idea what the issue might come from, has anyone ever experienced something similar?

It really is just the issue with a different network, if I plug it in at home it works perfectly.