A particular poster's threads/posts have all disappeared, and the PMs I had received from him have now been attributed to a different username, a user who hasn't visited for an unspecified time.

A shame, by the way, as I was keenly awaiting the next installment.

Hope all is well with the board and with the poster.

Everything's fine. Check your PM.

Edit: the "last visited" value in users' profiles is unreliable. Ignore that. It's a bug in phpBB that's existed in every version we've used in recent years. :-/ It's nothing to do with the other things you mentioned (explained in my PM.)