I just started using Vista backup ('Back up files', not 'Back up computer'),
and was running fine for a week or so before receiving the 0x81000006 error.
In my case, my backup destination was full (I still haven't figured out how
Vista determines when to do a full backup and when to do an incremental). I
moved the existing files to offline storage, figuring that would fix the
problem, then tried the 'Back up now' button to pick up where I left off.
Unfortunately, each time I tried to back up I still received the 0x81000006
error. After a few more dead ends, I figured I'd try a full backup - and it
worked. Actually, the backup is in progress right now, so I can't walk
through the specific actions I took in the 'Backup Status and Configuration'
tool. But, if you navigate through the 'Change settings' dialogs (where you
choose the backup location, types of files, backup frequency, etc.) there
will be a check box on the last page that lets you make a full backup right
now. That's the option that worked for me; hopefully it will fix your issue,
I'll keep looking into this to determine why I couldn't just continue with
my incremental backups. Likewise, if you find any new information, please
post it here. Thanks, and good luck.