I'm experiencing the same issues, both in my website as well as this page.  When I tried to reply to this post, I got the error message.  One thing I found in common betweein my end user (the one complaining about the issue) and my problem is that we both are using the same browser, IE 8.  The "funny" thing is, that the error is random.  If you get the error message and refresh the page a few times, it will finally enable the control and will let you type.