solved
i solved my problem by doing the following:
1. flash the latest twrp 3.0.2.0
2. try backup - failed
3. went to settings
4. tapped 'use default settings'
5. tried backup again - no errors
6. fixed!
Hope this help someone.
You went from an unofficial version to the official one I suppose. If this is the case you have to delete the .twrps file.
i solved my problem by doing the following:
1. flash the latest twrp 3.0.2.0
2. try backup - failed
3. went to settings
4. tapped 'use default settings'
5. tried backup again - no errors
6. fixed!
Hope this help someone.