yes i have root (SuperSU-v2.68) works great, and i really don't remember the "previous version" but is the last versions of exchange work (i think the backup is equal or previous to 8.6.9.8)
I have same bug i fix it installing titanium backup in previous version create a backup for apptadata from mail and fresh install 8.6.9.15 finally restore data from titanium backup :-)