I had the same problem so I find your post.I don't know if you still need help,I found the problem.
The situation is that google make the Service Account to be a real google account,so it makes the progress becomes other account try to add file to your google account.Then I create a folder and set folder's sharing to anyone can edit. And I can finally add file.
Hope this can help you! sorry English is not my native language,my typing could look wierd :(