I experienced exactly the same problem when using Firefox. So I tried using IE7, which worked like a charm. So I came to the conclusion that there was something with Firefox that was causing the bug.
After turning off all add-ons and turning them on one by one, I reached the conclusion that it was Firebug that was behind the bug. In more detail, it was when I had paused Firebug (right clicking the icon and pausing it) that the error would occur. If I disable it from the add-on preferences things seem to work, the same thing if I have it enabled completely.
Hope it helps you too. If not, I recommend you to turn off all add-ons and enable them one by one. That way you’ll (hopefully) find out what’s causing the bug for you.
