I have this same problem. The log indicates that the sql is being backed-up but it does not appear in the App/wpb2d/wp-content/backups folder in my dropbox. There are no indication of errors in the log. I also checked the backups folder under wp-content on my website, the only thing there is the log file.
-server configuration-
Wordpress: 3.4.1
php: 5.2.9
wpb2d: 1.4.2
I see that my server has the rather old php version 5.2.9 could that be a factor?
