If you read the warning, you'll see that content from derok.net is the problem, as they've been tagged as hosting malware. This almost always results from an image, and sure enough, that was the case here. I've removed the post in question, and now the thread is fixed.
I noticed when I searched for derok on the forums, I found 72 more posts, so a bunch of other threads will have the same issue - I'll find out whether we're best to filter out the website name or delete all the posts.
For future reference, this is the place for general forum issues:
http://forumserver.twoplustwo.com/55/about-forums/