Try a forward slash. Mine would consistently choke on a backslash, and choked in odd ways with several.
With google drive: Failed to connect: The remote server returned an error: (400) Bad Request. #2292
reported success too. For me, another oddity was that things only worked if Duplicati created the folder…
EDIT: I didn’t get far on figuring out the backslash oddities, but I suspect the access oddity is a scope feature to restrict Duplicati to its own files. This is in general a good thing, but may surprise people doing migrations (as I did to migrate into a subfolder to see what would happen on “Test connection” with various slash styles).
See Here’s the OAuth 2.0 scope information for the Drive API and below, and this post on how I think I set up.