I am having the same issue. I deleted the addon.d and then try to backup the ROM again and it gives me the same error. The addon.d is back. I then delete it again and try and the addon.d is back again. No clue why it keeps getting added.
[BACKUP STARTED]
* Backup Folder: /data/media/0/TWRP/BACKUPS/ZY2233DBZ8/Pure.2016-10-18/
Backing up System...
Error opening: '/system/addon.d' (Not a directory)
I:Error in Generate_TarList!
Error creating backup.
I:InfoManager saving '/data/media/0/TWRP/BACKUPS/ZY2233DBZ8/Pure.2016-10-18/system.info'
createTarFork() process ended with ERROR: 255
Backup Failed. Cleaning Backup Folder.
I had this same issue on my nexus 6P...what ended up working for me was disabling compression. I *think* TWRP/tar is having issues creating the tar with odd directories. Maybe try disabling compression and give it another shot