Docker on Unraid: Cannot authenticate to Kopia UI with configured credentials

Hi, I’m trying to set up Kopia on Unraid in a Docker container. The container is configured as

docker run
  -e TZ="Europe/Berlin"
  -e HOST_OS="Unraid"
  -e HOST_HOSTNAME="nas"
  -e 'KOPIA_PASSWORD'='password'
  -e 'TZ'='Europe/Berlin'
  -l net.unraid.docker.managed=dockerman
  -l net.unraid.docker.webui='http://[IP]:[PORT:51515]'
  -l net.unraid.docker.icon=''
  -p '51515:51515/tcp'
  -v '/mnt/user/appdata/kopia/config':'/app/config':'rw'
  -v '/mnt/user/appdata/kopia/cache':'/app/cache':'rw'
  -v '/mnt/user/appdata/kopia/logs':'/app/logs':'rw'
  -v '/mnt/user/media/kopia':'/app/data':'rw' 'kopia/kopia' server start

The “Post Arguments” are given as server start --insecure --address --server-username=admin. This is the command given by default, but I have removed the --htpasswd-file ... argument, as a) there is no such file and b) as far as I understand, this method of maintaining users has been superseeded.

However I cannot log in on the KopiaUI as admin/password, but I don’t understand, why not. Can anyone shed a light on this problem? Why can’t I log in with the configured credentials and where in the documentation can one find appropriate instructions? :confused: