The fact that you don't receive any content is because you log in to your forum and Digests won't send you messages posted before you last login time.
Try this and you will see that Digests is working fine:
- Create a fake user with a real email you can check.
- Enable the Digest for that user and select all forum and Daily delivery.
- Do not login anymore with that user... at least for a couple of days.
- That user should receive Digests email correctly...
Can you try this?
As I said... Digests and Subscriptions are not linked... they are different things.