I had the same problem, and I was able to get to the root cause. I had ‘subscribe to comment’ plugin installed, and had added code to comments.php [show_manual_subscription_form();] to allow users to subscribe without commenting. However, instead of adding it after the </form> tag (as recommended), I had added just before the </form> tag.
@swaroopch your solution would also solve the problem, but its a workaround. Maybe its good for you (and others who, like me, might find the page in future) to know.
Hardeep Singh
https://blog.hardeep.name