That you’re progressing to a new error is a good sign that the original issue has been resolved (likely by the ca-certificates-mono and cert-sync steps).
I think I saw that you’re running Duplicati 2.0, but can you provide the rest of the version number? I suspect you’re running into something like the issue below, but how to deal with it can vary depending on if you’re using 2.0.2.1 beta or a later canary: