Just started using Duplicati 220.127.116.11.1 a few days ago. Started on a Win 7 machine and Google Drive, works perfectly. Installed it on my Linux Mint laptop. I get an auth code from Google and select Test Connection. It just stays there and never fails. If I backup to a local folder, all is good.
I get the same issue with Chrome Version 74.0.3729.169 (Official Build) (64-bit) & Firefox 67.0 64 bit.
I have tried wiping Duplicati and reinstalling. I tried multiple Google accounts. I see in the Google account manager that Duplicati is an authorized app.