I am having the exact same issue. The 'backup' function for some reason does not work if you check off the device you want to back up to. Now, if you share the link from the "choose device screen" and then add the link to the other devices that way it works fine. The device will not be checked in the list of devices the 'backup' is going to, it just works as it is. If this ain't a bug then I don't know what is. I haven't seen a QA department that is so asleep at the wheel as the bunch here looking after the sync applications.