Sorry for the necro. We just ran into this exact problem and this topic led me to realizing what the problem was, so I wanted to post the solution for anyone else who searches for this.
The issue is that I'm going to guess you have AdBlock or AdBlock Plus enabled, and the commonality between all problem forums and threads you can't interact with is that they have "advert" in their name or topic title. If you disable your adblocker, it will allow you to interact with that thread again.
This is why some people with Chrome can inexplicably reply to those topics - they don't have an adblocker.