I'm curious as to how this turns out. I've also got an HD 8, 6th generation, that's having the same problem. Local saves work as expected, but I get the same 403 error. I have drive on my device as an app, and logged in, I've logged in through the silk web browser, nothing seems to stop the 403 error for me. I also have Terraria installed on my Android, it just occurred to me that maybe having that device linked to my Google account is interfering with my Kindle's ability to connect?Thanks. I did install Google Drive and logged in there but still get the same error. Whenever I click the Google+ symbol inside Terraria I get the same 403 error.
The end goal is to transfer the saves to a new Kindle Fire HD 8 because we are making a warranty claim. So if there is another way to do that it would be helpful to know.