So you are doing a backup consisting of just the files, or System image and files, or just System image?
If you are doing the files, since you can choose what to backup, maybe some folder/file is causing the problem. You could try skipping certain areas to see if it would complete and narrow it down later.
Do you have your backups scheduled?
You have a very large partition for Windows, what size is the drive the partition is on? But I think Windows uses copies of your data to make the backups, so it needs room to keep those. From what you say about available space, it does not appear to be limited, so maybe something has become corrupted.
Edit: Since you have already run a System File Check, perhaps you might consider a Chkdsk in case there is some file corruption.