The problem is that Google has implemented this in a way that is not working well for 3rd party tools. The logic is that only files Duplicati has created are accessible to Duplicati, which is exactly what we want. Duplicati cannot use your documents, pictures or spreadsheets anyway, so better shield them from access.
However, some things can cause the permission to be lost. I don’t know exactly what causes this, but many users have reported that files are suddenly inaccessible.
The problematic part is that Google provides no way to change the permissions, so you can never grant Duplicati access to files once the permission is lost. I have spent a long time trying to communicate this problem to Google, but they have other priorities.
Sadly, my attempts to fix things have cause Google to retract the original permissions, so it is no longer possible to obtain full access. Attempting to solve this with Google has so far only resulted in an endless reply of the same email responses.
If you do not have space to store the remote data locally, you can set up your own OAuth server, which will then let you grant full access to 100 accounts in the “Test mode”, which should be enough for your personal needs. It is quite a hassle, but I have documented the steps here:
Maybe we should investigate if rclone has found a workaround for the permission issue?