I just installed Snapshot, configured it for 2x monthly, and manually started the first backup. Nothing happened. Reset it to run manually. Nothing. Killed all non-WPMU plugins but Gravity Forms and Akismet. Nothing.
Because this is a strictly backend plugin, I realize there's not much anyone can do without a login; I have one ready to go. Just tell me where to send it.
Screenshot below, though, for kicks.