I'm having a similar problem, I think. I use this plugin (version 2.2.3) on 3 different websites. Website 1 runs WP 3.2.1, website 2 runs WP 3.2, and website 3 (the one which has the problem) runs WP 3.1.3. I'm trying to schedule in a WordPress update for website 3, but I don't think that's the issue as before I updated website 2 from WP 3.1.3 it was working there.
I don't know if it makes any difference, but all 3 are on the same server and are scheduled to backup and send the backup to my email address at 1am daily. I've never had a problem with it for websites 1 and 2, only website 3. To begin with it was working on website 3, but now it only occasionally sends a backup to my email address - it worked on Jun 21, Jun 22, Jun 25, Jun 27, Jun 29, Jul 4, and, strangely, today about 15 minutes ago.
I echo Christopher's message: all help appreciated!