Are you accessing the website from CasaOS or another computer? By default, Duplicati will only listen on the local PC and not external connections.
Where did you get this config screen and the image, is there more information on it somewhere else?
I think with network “bridge”, the IP is only for an internal docker network. So each container will act as a different PC on that network, which means it would be treated as an external connection for Duplicati. Can you configure network “host”? Once you have access to the UI you can enable remote access so it should work when you turn it back.
Edit
If you are able to add command arguments, this might also work:
Try to add DAEMON_OPTS to environment variables maybe? Otherwise look at linuxserver whether they have a way to specify start options.