It sounds like you’re now having the same issue as mentioned here:
In that case I believe the problem was that “localhost” (which Duplicati uses to connect to the GUI) was not being resolved to the 127.0.0.1 IP address.
You should be able to confirm whether or not this is the issue by running nslookup localhost. If it doesn’t return 127.0.0.1 you may need to add something like the following to your /etc/hosts file: 127.0.0.1 localhost