I cannot type anything in the reply box of a PDF comment

I am using Acrobat 11 on Windows 7 Enterprise.

I am trying to reply to the PDF comment of another collaborator this way:

1. I right-click the comment's pop-up note and select Reply from the context menu.

The original comment opens in a pop-up window, and a reply box is appended to it for me (See attached).

2. I try to type into the reply box, but cannot enter anything.

I have unchecked the Edit > Preferences > Security (Enhanced) > Enable Enhanced Security check box. Still, I am unable to type anything in the reply comment box.

How to fix this?