Umbrel Dreams is right:
Below a more detailed explanation:
Connect to the Umbrel with ssh (in my screenshot I used winscp)
1.) find App Directory
2.) find DockerCompose File
3.) add PROXY_AUTH_ADD: “false” in the App Proxy config
Restart the container (can be done in the umbrel webinterface).
Greetings
Thomas
Further informations(found the solution in this documentaion): umbrel-apps/README.md at master · getumbrel/umbrel-apps · GitHub