This is only appearing for this particular pm--I can start a new conversation with this user, and the reply box is showing for other conversations that he's sent me in the past.
What could be causing this? Does it mean he's left the conversation? He's still listed as one of the participants... not sure if that means he's an active participant, or that at one time he was a participant.
Along those lines, if I send a message to multiple people, and one of them leaves the conversation without replying, will they still be listed as a participant?
I have a Xenforo 1.4 forum. Very vanilla install, few addons, none that touch PM functionality..