Unfortunately not a solution, yet, but I just ran into this. Previously it worked as it should but suddenly it's not working anymore. Press the button and nothing happens.
I will let you know when I figure out what caused this.
I am having the same problem. It seems to be related to admin priviliges. Where I work, we do not automatically have admin priviliges, but have to switch user and login as admin. When I do that, the template editor seems to open fine. Not sure if that makes it a SW problem or my organization's problem...any thoughts?