Is this PC the Ubuntu server running Duplicati? If not, you are in above “another computer” scenario.
127.0.0.1 a.k.a. (localhost) means you can only get there locally unless you loosen up some security.
IP protection from firewall will help you, but if I read above right, you’ve only got LISTEN on loopback.
EDIT:
Note that should be enough to allow the curl line to run from Ubuntu server (I hope, depending on its networking setup). These days Windows PCs have curl too, but yours lacks Duplicati unless it’s same system. If you need to go across a LAN or something that’s not localhost, you need to arrange for that.
Settings in Duplicati right at the top of the first image shows how to do that – iff you can browse locally, otherwise you’ll have to arrange for it at Duplicati startup (which is what – boot time? User login time?).
For start at boot time under systemd, you’d add options into /etc/default/duplicati such as the following:
–webservice-interface: The interface the webserver listens on. The special values “*” and “any” means any interface. The special value “loopback” means the loopback adapter.
The above gets access from other systems, but you must do it by IP. If you do it by Ubuntu’s hostname
–webservice-allowed-hostnames: The hostnames that are accepted, separated with semicolons. If any of the hostnames are “*”, all hostnames are allowed and the hostname checking is disabled.
and I see the manual doesn’t yet reflect the latter, which was added to help stop DNS rebinding attacks.