Yes, I am using 2 different subdomains, e.g. "repetier1.domain.com" and "repetier2.domain.com"
I am using Traefik as a proxy. It should already be able to handle ws/wss connections by default, but I also tried setting the headers manually which you provided. Both unfortunately don't work.
The Chrome dev tools also say that the connection to the web socket is working (for the UI) and messages are being sent over it (applies to both instances).
Here is my Traefik config for the routing to the raspi. The config for the other instance is mostly similar, only a few entries changed, since it is running on the same machine as the proxy. Note: The configuration for the UI was already working for the "printer" setup. After you answered, I also added the "printer-wss" setup but that apparently also doesn't work.
yml<br/><div><div>http:</div><div> routers:</div><div> printer:</div><div> rule: Host(`repetier1.domain.com`) && !PathPrefix(`/mod/front`) && !PathPrefix(`/modules/front2`)</div><div> service: printer</div><div> entrypoints: websecure</div><div> tls:</div><div> certresolver: le</div><div> printer-wss:</div><div> rule: Host(`repetier1.domain.com`) && PathPrefix(`/socket/`)</div><div> service: printer-wss</div><div> entrypoints: websecure</div><div> tls:</div><div> certresolver: le</div><br/><div> services:</div><div> printer:</div><div> loadBalancer:</div><div> servers:</div><div> - url: "http://192.168.2.157:3344"</div><div> printer-wss:</div><div> loadBalancer:</div><div> servers:</div><div> - url: "http://192.168.2.157:3344/socket/"</div></div>