Unable to change WordPress address and Blog address

I am trying to move my WordPress 2.6 blog to a different part of my site. I am following the instructions given in the Codex (and I have done this before successfully with an earlier installation of WordPress). But now, whenever I change the WordPress Address (URL) or the Blog Address (URL) in the General Settings page and then click “Save Changes” I am immediately pushed out to the WordPress login screen. I log in again and find the WordPress and Blog addresses unchanged.

Any and all help appreciated!