The logs from this are already cleared on my server, but when I sent a private message (standard Create/Note with a single recipient who also has a Mention in the tag property) from my custom software to my account here, I received a 500 error saying something like "cc is not iterable" (though I don't know the exact phrasing). When I included cc as an empty array, it started working. Not sure whether it was missing on Create or Note, maybe it was both.