WebException The operation has timed out sounds awfully similar. Answer there might apply, however the one here is the attempt to find the Internet address of ingress.duplicati.com.
Transport over the Internet is done by address. Names are looked up first to find where to go.
I can reproduce this issue if I have my computer disconnected from Internet at end of backup. Possibly your situation is different. Outages can occur beyond the local end of the connection.
If you click on the link you posted, and your browser can’t get there, Duplicati might not either.