The on-prem could impact it, depending on how current the install is. Usually I'd knock of the api bit to solve that problem.

The issue is that you're trying to go one level too deep, hitting the Shared Documents folder. This should work for you:

Code:
let    Source = SharePoint.Files("http://xxxxxx/Department/Sales", [ApiVersion = 15])
in
    Source
If not, try this:

Code:
let
    Source = SharePoint.Files("http://xxxxxx/Department/Sales")
in
    Source
From there you need to filter down into the required folders, but you always have to connect to the root folder to begin with SharePoint. (Frustrating design decision there!)