For a while I could not make a comment on anyone's blog post. It was very frustrating. I could not even reply to anyone's comment on my blog.
I searched the web but all the suggestions did not help. Then I took a chance and experimented on my blog settings.
Finally I found the cause. I had Ghostery installed to prevent some advertisers from tracking what I looked at on the web. It's not just the advertisements, which are a nuisance; my privacy was being attacked.
Anyway, by adjusting the settings on Ghostery to "Trust Site", it allowed me to comment again.