I think this is the source of the problems:
The name you set on a folder is just a label. All files and folders have an internal ID, so you can easily have multiple folders with the same name, although it is really confusing to browse it from the Web UI.
When you log in with via the OAuth handler, and choose “Google Drive” you only give Duplicati permissions to see/touch files it has created itself. Thus if you create a folder from the WebUI, Duplicati has not created it and is not allowed access to it.
“This is not a bug, its a feature”. Specifically, it is done to prevent a rogue Duplicati from wiping your Google Drive (extremely unlikely, but that is the idea).
If you do not like this, and want Duplicati to have full access to your Drive, you can use the button “Google Docs” (stupid name for this, I know) on the OAuth handler page. The generated AuthID here will grant full access to your Drive.