Digest sent an empty email

What does the template preview show you?

Every user has his own timer for every send interval. They run independently. If you have daily intervals and you do not trigger any notifications for two days, and then you trigger one, then they should all fire, but they're likely to get out of synch again over time. There's no point in trying to understand that, unless you intend to spend a couple of days to truly get to the bottom of it. I'm pretty sure that the send intervals work correctly. Please stop reporting these observations (especially if you don't add your expectations). The goal of a Daily send interval is that each user does not get mail more frequently than once in every 24 hours and at the next cron run once a little more than 24 hours have passed since the last send. That's the expectation and nothing else.

The empty mail is another matter. We need to investigate that. Create a new user with an update subscription of a different type (e.g. different content type) and a send interval of ASAP. Then you can change your test node and trigger cron manually to get an immediate response. Clear the template for that content type as well as for the digest, so that you get the default templates. Try sending non-digest notifications first. If that works, then try sending digest notifications.

No follow-up?

Or should we close this after #1172646-15: Subscriptions for D7?

Congratulations and best wishes to all of you.

It doesn't make sense to do beta testing on a live site where you cannot post and experiment freely. I have no idea how your site is configured and incidental bits of information can't provide a coherent picture. To make this meaningful you'd have to start from scratch with a virgin site and be able to try things systematically.

So, thanks for all your help so far, and you take care of your wife and child now.

See #1402222: Blank email subject and body with HTML Mail module -- do you have the HTML Mail module (or some other third party) installed?

Thank you for the list — I don't see any suspicious item.

Thank you for the offer, too, but this goes beyond what I can do for an issue that seems to be rare.

So, citing from #5:

To make this meaningful you'd have to start from scratch with a virgin [test] site, install nothing but Subscriptions, check whether it works*, and then work towards your production configuration and find out at which point (which module/configuration/whatever) it breaks.

* if it doesn't, then I have to come in again, but if it does, then you have to pursue it...