Ajax error with status code 200

I have a very long text say of about 1L characters in one of the posts. When user tries to comment on the same, Alert box pops having following details:

 An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /system/ajax
StatusText: OK
ResponseText: 

But when i reduce the text to nearly 93,000 characters, everything works fine. The dblog says "Invalid form POST data". The comments right is given to only registered user thus there is no caching problem also. Please reply.