Mac 10.6.8: Time Machine could not complet the backup.

For my iMac, running v 10.6.8, I have been getting this message: "Time Machine could not complet the backup. The backup disk image 'volumes/xxx.sparsebundle' could not be accessed (error -1)." This has been happening for a few months. I think I occasionally have a successful backup, but this used to all happend automatically, behind the scenes.

Now going through a manual backup process, but slow.