If your in a question, bug, or feature request, when you click send PM to someone who has responded in that thread, the PM system works fine.
If I have a user who requested a boxset in my fan art, and I want more details. I click send PM, and all I'm given is a pop up window with a black X to close.
I tested this in several fan art threads, if I want to send a PM to anyone that has replied in a fan art thread this bug happens.