Honestly Ngaeg, I'm not sure. Try the newest version that I uploaded (everyone should update).
This version currently does everything completely fine (and now even deactivates properly when the window is inactive). I'm not 100% sure if the deactivation matters if you are not on english however since it uses "Neverwinter."
I've also noticed that the anti-afk seems to not work. I'll work on that soon as well.
Ngaeg, possibly try editing this line
ImageSearch, buttonX, buttonY, foundX, foundY-50, %A_ScreenWidth%, %A_ScreenHeight%, *60 joinButton.png
to have the 50 be a slightly larger number? 50 was chosen at random. Basically it is there so that it doesn't try and click the other join buttons.
EDIT: If anyone is having problems with things not including leaving party, it is either your images... or there is some sort of difference between our computers/games that I don't know about.