You just need to edit an existing config file in a Duplicati folder per How to: Configure network tracing.
It’s a bit more than copy-and-paste-at-bottom, because new content goes in original <configuration>.
This will make a big file, but it’s a view on the application side of the TLS encryption, which is helpful.
I’m not very familiar with this, but I think you want to read HttpWebRequest.GetResponse Method and
https://github.com/duplicati/duplicati/blob/master/Duplicati/Library/Backend/GoogleServices/GoogleCommon.cs has a couple of spots that might wind up in GetResponse() with your error details.