Sorry this is a bit late. I’m not sure if this will work for you, but I found that SF wasn’t recognizing a X 10 Plus backup from Dropbox, but managed to get it going by doing the following:
- Create a new backup on Dropbox. This creates an Xperia 10 II folder there.
- Log in to Dropbox from a browser, and delete the newly created backup and copy the one you want to restore into the Xperia 10 II folder.
- Go to Settings>Backup, and you should be able to restore it