I just tried it and it gives me the same problem. So it looks like you've found another one of the many, many bugs that exist in newer versions of the board's software. There's not much we can do about that I'm afraid, you'll just have to use the button at the top-right-hand corner of the thread instead. Click on your username in the bar near the top of the page, and click "My Settings" in the drop-down menu that appears. In the settings page, click the "Notification Options" tab on the left. Where it says "Notification method to use for replies to followed topics", check the box under "Email" then click "Save Changes".