This is an intermittent problem that we're still tracing down. Restoring 2 or more documents at start-up seems to be the most common way to see this problem. The root issue is that an extra button is being created (for some reason) which is actually hiding behind the actual tab, and that is stealing the button clicks. As a temporary workaround you can simply grab that document tab with the mouse and slide it a little bit to the right. You'll see that "shadow" button, and after that the doc tab buttons will work.
