Note:I could not reply to original post on these forums.
I had below error on a new CentOS 7 install.
Tried all suggestions found online regarding installing and reinstalling “ca-certificates-mono” but I already had it installed and nothing else worked until I came across this how-to SSL TLS support in Mono · duplicati/duplicati Wiki · GitHub
and after issuing this command “sudo cert-sync /etc/pki/tls/certs/ca-bundle.crt” I was able to continue with my first backup. Hope it helps someone!
Failed to connect: The server certificate had the error RemoteCertificateChainErrors