I have been following this post with my own additional problem of the iCloud showing the wrong user space. None of that Networks Setting Reset thing helped and nor did the logging in and out of the iCloud account. This is what has worked for me.
Problems: 😟
1. iCloud showing almost full with an unknown 3.51GB backup data that cannot be found on iPad, iPhone and iCloud.com. It was almost impossible to delete.
2. My iPad showed a 0KB backup size. When I tried to delete that backup and run once again, the 0KB size remain constant but the next backup size was 3.3GB. It simply showed me that I am out of space. I never had a free space of less than 2.5GB.
3. I could not delete those apps from my iCloud backup preferences showing an error "The backup could not be deleted right now". I was so stuck.
I believe most of the people do have the problems as mentioned above. I would ask all of you to try this before restoring your phone and settings or upgrading it to iOS 9.1. That's even more tricky, so.
Solution: 🙂
1. My iPad was showing a 3.3GB backup with "No data" shown in all the apps. I deleted the backup first and turned my iCloud backup off.
2. I knew which apps were having too much data previously. I ran a full backup on my PC and deleted all those space hogging apps. This made them disappear from the iCloud apps backup options.
3. Tried backing up the iPad with all the remaining (no third party other than GMail, Facebook and WhatsApp) apps or say, native iOS apps. I doubted that it would stop after sometime but voila! It completed.
My advice (from my experience) is to: 😉
1. Take a full backup of third party apps and documents on PC via iTunes. You may click the Apps tab, select those thrid party apps individually and backup. You may reinstall all those third party apps later, once the iCloud backup is completed successfully. Remember, there is an option to backup all those apps' data via PC/Mac iTunes and you most certainly won't lose any of it. DO IT FIRST!
2. Perform an iCloud backup having no third party apps (you may allow the non-space hogging apps to remain in the iCloud preference) installed on the iPad or iPhone. Let it run completely. It will be only native iOS apps with a few apps like Facebook, GMail, WhatsApp only. This somehow overwrites or deletes or does something good to that buggy unknown backup.
I had a very slow internet connection but I believe it will work well for users in other countries.
I hope this helps all the concerned people. Kindly let me know if it worked. Thanks! 🙂