I had a weird experience earlier today: going to the link in a comment approval email produced a white screen. The only thing I've changed since I last approved a comment was updating to 2013/07/04. I've now upgraded to 2013/07/05, but haven't had any comment approvals since to check. However, I don't see any references to this in the changelog, so it might be worth looking into.
Please note that I can approve comments via the admin comments panel - it's only the page that the comment approval email linked to that didn't work.