I am also getting that 404 error. I cannot add new posts or edit any of my posts. My theme is Red light 1.2.0. I did not upgrade to 3.0. This is my first time using WordPress.
My first issue was that I could not get my WP Audio Player to work even though it used to work perfectly fine. Then my issue was that I kept getting blank pages when I tried to upload new MP3 files. Now I cannot update Posts or make new posts at all without getting the 404 internal error.
What to do? I can’t even change my theme without getting the 404 internal error!