When a user activates an account, it goes to an almost blank page

I thought things were going well with my Multisite install, but I've just discovered a problem.

Once a user has signed up and clicks on the activation link in the email, it goes to a page that has the header on it, but nothing else. The user is not activated by going to the page either.

This is the activation link shown on the email, which will give you an idea of what I mean:

Any ideas what could be causing this?