Hey, same problem think I figured out what to do NO THANKS TO APPLE - backup by wire to iTunes first than on wifi, go to settings/general/usage/manage iCloud and nuke your existing iCloud backup and move up a few directories and do a fresh iCloud backup - worked for me- there is some bug in Apple's transitioning backup to 9 - maybe a rogue app, whatever, but my iPad zipped in a current backup.IPhone backs up to iCloud ok but iPad Mini2 just won't do it
Posted what I did below but think (this is do awful for Applesomeness!) anyway, do a wire/sub backup to iTunes then move to wifi and go to settings/general/usage/ then down to manage iCloud storage- there nuke your existing backup then do a fresh backupI selected reset all settings and reset network settings. Switched off iCloud, switched it on again, untoggled calendar, noted etc ruthlessly went through and deleted many photos and was FINALLY about to get a backup going.
Estimated time - 19 hours!!!
I'm at my wits end with this thing!