Yeh I have an iPhone 6s coming next week and without a backup I will have to transfer everything over manually. I thought those days were behind me! Just did a search on google for this keyboard data and it looks like it's been a bug since the ios8 beta in 2014 and has something to do with your recently used emojis. If it is this causing the problem then I don't hold out much hope for a fix.
Man, that's ridiculous. That would mean a ton of people are walking around now with a phone that can't be backed up.