SignalR in Synology NAS in deployed with VS Studio and Docker Hub

Question

I deployed an ASP.NET Web API application in VS Studio to Docker hub and run it on a Synology NAS.So the normal endpoints work fine, but I use a SignalR connection to my Blazor Wasm clients. This connection is not working anymore, but I don't know why...

I use the synologie.me domain with LetsEncrypt certificate. In the reverse proxy settings in my DS I also declared the websocket headers:

Upgrade --> $http_upgrade
Connection --> $connection_upgrade

Does anyone know what I have to change too?

Regards Samuel