iCloud has space but iCloud Backup says that I don't have enough to complete a backup

This could easily be explained as the 120 MB of used data is likely for the data all the apps store in iCloud. Contacts, calendars, specific third party apps can have storage in iCloud. You can determine this by looking at the Manage Storage pane in settings app (see below).

The warning you describe comes up when the expected size of the next backup is greater than the available free space.

You can reduce the size of the expected backup on iOS (updated for iOS 12) as follows:

• open the Settings app
• tap your AppleID/iCloud avatar (first item, top of list)
• tap iCloud
• tap Manage Storage
• tap Backups
• tap the name of the device in question (it usually says this iPod touch, this iPhone or this iPad to help if you have many devices connected to iCloud)
• look at Next Backup Size
• toggle off any unwanted apps from the list below

If that is less than the available space, you may need to contact Apple support to have your account fixed on the server side. In most cases, you can toggle off the largest few items and then attempt a backup when the estimate is less than the available space.

Once the backup is complete, you can tune the backups or manage your space as needed on iCloud.

In my case, I have the problem that iCloud estimates my data usage to be 10.9GB, while I have chosen to back up maybe only 1-2GB. If I turn, for example, camera roll off or on again, it doesn't matter.

iCloud estimates this wild, high, far too much space to make a backup, and I even deleted my previous backup and started new, to be sure to 'reset' everything, but it doesn't help!

My iPad only uses 2.2 GB, I have 2.8 left for the phone, and select 'no backup' for most, but the mail and a few programs like Skype and so on, altogether far less than "10.9" GB, probably only 1-2GB.

This problem seemingly happened before, the iCloud backup grow out of control, like a classical Windows installation.

However, I now see that, as I deactivate more and more smaller applications, of a few kb size of their data, that the total usage decrease much more than these few kb which are stated for each app.

This means that Apple seemingly underreports how much data they backup, from each app, or maybe the system counts too much, I don't know.

It is not possible to deactivate all the apps at once, in the hope to get down to the 17MB which is stated on the first page, (e-mail and settings), so there will be a lot of job to make this work again... :-/