Pvt RCPT msg with empty 'to' and 'cc' fields
Per Mike's response... Looks like a bug in Hubzilla's ActivityPub driver. I just tested and ActivityPub messages with private recipients are going out with empty 'to' and 'cc' fields and are therefore getting rejected. I'd recommend opening an issue in the framagit repository. You can use my text in this comment to explain it. I'm not certain I'll be able to take a closer look any time soon.
[Edit: not certain if that's the issue since I think we send those as bcc. In any event the messages appear to be getting rejected.]
[Edit2: There's a setting in your ActivityPub app which by default doesn't send to folks in privacy groups. We needed this long ago. I don't think we do now so you probably want to toggle it. I tried this and it is now sending a 'cc' field that looks correct but still getting bounced on pleroma and mastodon.]