Q: How is it possible that my icloud storage shows 1,8 GO free, but then I m not able to backup because "not enough storage"?

Hi christophefrombudapest, 

Thank you for visiting Apple Support Communities. 

It sounds like there is not enough space available in iCloud to back up your iPhone. 

You may find some of these tips helpful for freeing up enough space to complete a back up:

Get help creating an iCloud backup - Apple Support

I can't back up because there's not enough storage available.

If your iCloud storage reaches its maximum capacity, you can take any of these actions:

• Upgrade your iCloud storage plan.
• Create a smaller backup:
  • Select which apps to back up. Include only the apps that contain important data.
  • Delete unwanted photos and videos before backing up. You can also save the photos and videos to your computer before deleting them.
  • Turn off backup for Photo Library (or Camera Roll in iOS 7), especially if you already back up your photos to your computer. (My Photo Stream isn't a backup of your photos.)
  • Delete old messages from your device, especially messages with attachments. To delete a message, swipe over it, then tap Delete.
• Delete items that you no longer need from iCloud. You can save them to your computer before deleting them. For example, you can:
  • Delete old backups, such as backups for devices you no longer have. 
  • Delete app data or documents.
  • Delete old email messages, especially messages with attachments.

Best Regards,

Jeremy