No NPC around, the PC was entering the trigger alone. This is really odd, since after further investigation it seems to affect only certain triggers (I have triggers like this in many areas) and sometimes even only specific parts of it (i.e. if you clicked or entered one end of the trigger, it would work fine, but if you tried it with the other one, it would be interrupted).
While I don't know the solution, I used the workaround of firing this conversation with the PC itself, which isn't a problem, since it was supposed to be more of a menu rather than a conversation anyway. It seems to work just fine this way.
The only problem is that the conversation won't end if the PC moves away from the trigger.