Very weird, never seen this before. I think the administrators are already aware of the situation

Okay, I did some thorough investigation into this, and found that it is from a new update by Microsoft.

This pop-up only appears to show in Internet Explorer, it does not come up in FireFox.

It isn't a possibility for this to be the thing that "hacked" anyone. As the entire program sends and recieves details only on your own PC. So it's not related to the "rishi90 got hacked" situation.

I had this pop up once while using FireFox but I did not get a screenshot so it isn't simply restricted to Internet Explorer. I simply clicked cancel and I stayed logged in, it didn't seem to having any effect on anything. It was in the Videos And Screenshots section.

Well I now know that it was caused by that signature we used to have that shows your stats in a picture.

So that's why it only popped up when you were in a certain place. I would assume that if you don't login to it, you won't be able to see the content of the picture, but at the moment the picture isn't working anyway, and that's why you don't see a difference.

When I had this I was using google chrome
and also in the video and screen shots section.

admino already explained why this was happening in a fair bit of detail.