It's a issue i've been having since i started playing.
When i open up my friends list and click on sombody, it will show the chat box as minimized.
I cannot open the chat box by clicking on it.
But after that anyone else i click on, it will open up a chat box and works fine.
The way i learned to solve this is to click on a random person, then click on whoever i want to talk to.
I use firefox.