404 Error for Child Pages

Any time I make a page a child of another page, when I try to load it then I get a 404 error. If I make it no parent it works fine.

I’m currently using WP-Creativix theme but have switched it to Twenty Ten with the same result.

Attempted to reproduce it on another installation, on the same server, and everything worked fine on that one.

I’ve got no active plugins on this installation.