One of my multisite network subsites had a number of subscribers. At some point, possibly when I upgraded the plugin to the current version, they lost them all - the list was empty. (They reported this at the same time as the other issue - the Subscriptions menu is no longer visible in the Control Panel.)
Have you ever heard of this happening? Is it possible to drop subscribers without someone actively deleting them? Is there any way to find out who the subscribers were (the admin had no csv file and has not kept the confirmation emails)?