I don't know what is causing this and been hoping David, would notice. Is it possible you need to clear out your back history and cookies & such on your computer. I used to get such pop-ups with my older computers and often clearing out the history and etc.. would improve performance. What operating system are you running?
I'm on Windows (I'm not very computer savy) It was just odd that it came up only when I tried to message someone. I thought maybe I had to be a "paid" member or something.