Sorry, wasn't sure where to post.
They are in german because I'm in Germany. This will appear randomly instead of a thread I'm viewing. The URL changes as well. When I press back, it doesn't go back to the thread but back to active or wherever I was before. Then one time, pressing back didn't work as the page just kept redirecting. I had to close the tab and start new. I've enclosed the 'history'.
Hope this helps because these pop up ads are so annoying.