You can try removing the computer, then repairing the backup database again and running a cleanup. Then add the computer back as a client.
Failing that, the
Home Computer Backup and Restore technical brief tells you where the backup database is. Follow the instructions for backing the database up, but instead of copying it to external storage, delete it at the appropriate time. This will fully reset the entire
backup database.
I'm not on the WHS team, I just post a lot. :)