Uploading to S3 is not possible for me anymore. This worked fine in 4.4.4. As you can see in the "Upload..." menu item is grayed out. The S3 user has ListBucket permission for the whole bucket and GetObject, PutObject and DeleteObject for the forums/* path, and no other S3 permissions. See attached screenshot.
Change the working directory to the bucket instead of / and the button will be enabled. While you have expanded the folder, the target folder for uploads is still / while you can only upload files into buckets.
You are correct, when changing the working directory the Upload option becomes available again. However I feel that this user interface is counterintuitive. For example, when right clicking on a folder and clicking upload, I would expect the file to upload to the selected folder, not the window's working directory. Anyway thanks for your work on this excellent product.