I can only guess why folk continue to try MSFT's backup programs. I stopped about 10 years ago.
I have a 3 year old Alienware desktop running Win 7 Professional. I have never had any major problem including running weekly backups to a second internal hard drive using Windows backup. About 3 weeks ago the backup failed and produced the mentioned error code. The description of the error is misleading and makes you think that you have insufficient room on your backup drive. I subsequently replaced the drive with a larger one but the problem persisted. I then researched (including here) the error code and found "solutions" that were all over the map including some frankly worthless responses. The most frequently described cause is attributed to lack of space in a 100 MB "system reserved" partition on the boot drive. When I checked my drive this partition was not present. I had an OS partition, a Recovery partition, and a 16 MB unlabeled partition. I thought that perhaps the 16 MB partition was supposed to be the 100 MB partition and, because it was too small, was causing the backup problem. I then took my computer to Data Doctors. They were unable to enlarge the 16 MB partition but created another 100 MB partition which they said would solve the problem but, you guessed it, it didn't. At this point I either need someone to solve the problem or start using a different backup program. Thanks.