Can you login to your control panel and see what quota you are using (used space vs allowed space) as we think it can be related to using up all the web space which can cause the database to stop.
Login to FTP and go to wp-content/plugins/wponlinebackup/tmp/ and delete all of the contents of this "tmp" folder (but not the folder itself.) This will remove the backup files that might be taking up the space.
To be able to backup your blog you'll need enough free space to do so. Usually around the same amount as your blog. So if your blog is 50MB you'll need at least another 50MB free space.
Also, have a look through FTP and see if there are any large files bigger than 50MB that might be getting backed up. The online vault will probably reject these if it sends the backup over 100MB anyway so you can exclude them on the General Settings page. Excluding these will also reduce the space used by the backup.
And lastly, if you've ticked "Backup the WordPress parent folder" on the General Settings page. Try unticking it before another backup. Sometimes people tick it without knowing what it does and it is only really for use in special circumstances and should be left unticked for most blogs.
Hope this helps,