I've had this problem happen on two different blogs.
The first backup always seems to work okay. But when I try to do a second backup, I get the message that says the backup was complete but there is no second backup.
I see two directories:
The first directory contains one file index.php which is empty.
The second directory contains another directory (20100705-1801) which is the very first backup I did but no subsequent backup. I expected to see another directory (20100706-1301).
In the Bei Fen control panel, it shows this second backup has been done....but it doesn't exist.
If I delete the plugin and delete the two backup directories, and reinstall, it works again but as before, only the first backup works. Any attempts to do another backup (using create backup) doesn't work even though the control panel message is: Backup has been successfully created! 16.75MB.
Using Version 1.4.2
Wordpress 2.9.n? and 3.0