This has been driving me up the wall as well (don’t get me wrong: I like Duplicacy very much!) - every time I open my laptop, there’s this error window. And every time I work off-line on my laptop, there are more error windows. In the end, I have to quit it to work (and then forget to launch it again when the network is back again).
They may not be the exact same issues, but it’d be nice if Duplicacy would not complain if there is no network. BTW, there’s a way to find out when the network comes back up - that’s why Safari refreshes its windows automatically. Very convenient.
(macOS 10.13.4, Duplicacy 2.1.0)