The inserted "document.writeln" line found by AndrewAMD would indeed redirect clicks to a script on om.qqtx-me. But this line isn't in the forum source code. I also don't get that popup after deleting the cookies. So the source is apparently not infected, at least not now, but possibly the cache.

I will clear the cache now. Please report when you observe the popup again or something else that is unusual.