Moderators can reply to closed threads only where they moderate.
It basically means that most probably they don't have moderator permissions in forum where reports are created.
DBTech has a mod which enables automatically setting all moderators to moderate forum(s) we choose (
https://vborg.vbsupport.ru/showthread.php?t=281850 - just not sure if you need pro version or it works in free version).
I just mention this as reminder, nothing else. When we have admin rights, we tend to forget what others can and can't do. I do that sometimes. Too often actually.