It seems a lot of people over a massive number of blogs have the same problem and hopefully I found the answer.
If you are using Firefox, go to your History settings (Options > Privacy)
If you use the option 'Remember history', there should not be any problems.
If you use the option 'Never remember history', you probably will have troubles posting comments.
In this case, use the option 'Use custom settings for history' and make sure the option 'Accept third-party cookies' is ticked (the other options are then up to you).
You will then need to restart Firefox.
I've reproduced your issue successfully by unticking the option.
Please let me know if it fixes the issue. thanks