The reason you are getting the message asking if you are sure you want to leave the page is that we always show that if the reply composition box is open, in order to make sure the user knows that if they leave the page the message they started typing (or were considering posting) will be lost. Thus, there is no issue there... that is expected behavior.
For the other issue - the white box- my guess is that the page simply did not finish loading, since I see no WYSIYG toolbar. That could be bandwidth issue on your end (if your internet connection slower than normal) or it could that something is interfering with the page load (typically, that could be a script you might have installed on your site).
I will test on your site in a bit to see if I can see anything obviously wrong.